My melotonin and sleep medicine isn't working anymore.

If you can get a consult with a sleep specialist they could help you with an overnight study and possibly a prescription.
Your doctor can order a home sleep study as well.
I just completed mine and waiting for the results.

It is possible that it might just turn around, but I would be betting against you...sorry. You have apparently gone on for some length of time needing sleep aids, and I can safely assume that prior to being prescribed those aids you were experiencing poor sleep for some time. So, all-in-all you have disordered sleep. The brain 'habituates' to pretty much any change by adapting. If you smoke, it stops making its own nicotinic receptor chemicals in its synapses. As long as you routinely up your serum levels of nicotine, it doesn't need to provide it itself. So this tells us that the brain adapts. It has adapted to your various prescriptions.
Have you ever investigated CBT or other forms of psychotherapy? Maybe you need to get something serious, and always pushed back down the hole, out into the open and deal with it. Literally get it behind you, not always in front of you. Your mind is active with....something. What is it?
Lastly, you're in excellent company if you're over 50 and can't sleep well. There are zillions of us. I can post how I manage to get two or three decent nights of sleep a week if you are interested, but there are no guarantees, not even for me. I can go two/three weeks with terrible sleep...just did.

@marigest
I experienced 5 years ago what you are now experiencing and it was a journey I wouldn’t wish on anyone. When Trazodone stopped working after so many years, I tried to sleep without a drug and developed an anxiety disorder because I could not sleep more than 2 hours and was literally falling apart. It cost me my relationship with my daughter. It took me several YEARS with intermittent trials of other drugs and OTC supplements before my body would allow me to sleep 4-6 hours without taking anything. I read an article once about an Emergency room physician who routinely took Ambien due to his schedule and finally realized he could not fall asleep naturally. He said it took him 5 years to heal his body and finally sleep naturally. My journey has been similar. Of course, menopause probably played a role in my situation as I was 66 at the time. Working with a sleep specialist would be the best idea so someone can guide you through the transition. I agree with the reply from @gloaming. It IS possible to convince your body that you no longer need a sleep drug but it is a very stressful journey and if you are over 50, you may find that your sleep will never be the same as it was pre-sleep meds. I would like to hear how @gloaming manages to get 2 or 3 decent nights sleep a week. I feel so blessed now when I sleep fairly well without a drug and it is still getting better over time. It’s been a harrowing 5 year journey.

So, my Gloaming friend, how much of your sleep problem is related to the darned CPAP? I have tried two different delivery methods. My sleep doc tells me I now need to add oxygen to the CPAP, but feel that would be useless if the devices are causing poor sleep time. Just about ready to throw in the towel on CPAP.
I have mild apnea and mildly low oxygen during sleep. I am a really good sleeper. At least I was untill I started down this CPAP trail. If the results of this treatment only knocks a couple of years off my life, I'm okay with that. I'm a quality for life trumps quantity kind of person. So there...I've vented.

Okay, as I said, I find I have to clear my mind of any impending doom/obligations/problems/relationship issues/financial troubles/...,or whatever it is that makes the mind want to ruminate the instant, and while, it is awake. Unresolved, unattended, neglected, put-off-until-later/procrastination....these things will always gnaw at a mind with the least bit of a conscientious nature...which most all of us coming to such health forums are likely to have. This requires fortitude, learning, time, energy, sometimes the enlistment of help or expertise, and most often.....a plan. Living hour-to-hour, drifting, never having a plan for the day, is a recipe for troubles down the road. Everyone should have a list of to-do's, and they should be actively completing them and stroking them off. This practice builds self-respect, confidence, and a sense of autonomy (google Julian Rotter's Locus of Control model and see why kind of personality you are, someone with an external locus of control or someone with an internal locus of control. Which if those do you think you should be, or which orientation to life's obligations and problems, should you want to have to be a confident, autonomous, and successful human being?)
From there, with a quiet mind, one free of guilt or unmet obligations, you can fall asleep. Now it's a matter of staying asleep. This is what I use, but sparingly (as I have said previously, the rest of the time I tough it out, usually three nights running as a minimum to build back an intolerance, or a sensitivity, to whichever sleep aid I plan to use that fourth or fifth or sixth night). Then, as my normal go-to soporific, it is either one or two 2.5 mg gummies of melatonin. That's it. No warm milk, nothing in my stomach if possible, an empty bladder, a cool bedroom, I use the moldable soft silicone ear plugs because my dear wife sounds like an idling diesel locomotive when I go to bed, and I fall asleep 'building something, or planning a project' in my mind. Sometimes doing simply arithmetic helps (you'd be surprised how dividing 1500 by 23 can put you to sleep in a few seconds).
I don't want this to become a tome, not here and now. No sleep aid I used lasts more than about five hours. From there, it's only hope working for you. Sometimes that dose of melatonin lets me sleep for close to 8 hours, but that's one time in maybe four shots at it. The rest of the time, I might get 6 hours, or close to it. It's better than four or five.
The other soporific I use is prescribe Zopiclone. It's a small oval blue tablet, and I cut it into halves and then quarters and try to use it maybe once or twice a month. Again, it sometimes does the trick, even at a paltry 1.5 mg quarter tablet, other times I take half a tablet. It, too, has a short half-life because it is metabolized quickly by the liver, perhaps 3 hours. So, it can get you close to 5 hours, and then you hope your calmed mind will add the rest of the bliss you need.
One final comment: you will sleep better when you are physically tired....spent. This means doing some considerable physical activity during the day....never too close to bedtime, so not within perhaps an hour and a half of bedtime. But going for a brisk walk during the day, splitting cordwood, cleaning out the garage, doing heavy weeding, digging trenches in your raised beds and filling them with your compost that you turn every two weeks (yet more physical activity)...even slow, plodding stair-climbing if you live in an apartment and don't garden...you'd be surprised at how much it helps you to sleep.

I took to the CPAP surprisingly well. I didn't like that I needed it, and it was weird the first couple of nights, but I adapted easily and have used it all but maybe two nights in seven years. The sleep lab had me correctly titrated, and the machine has done the trick. When I nap, which is like three times a year, and forego the CPAP thinking I'll not need it for the 20-30 minutes, I'm always proved wrong....I fall asleep but instantly awaken snoring. I have to use the machine....period! I never snore while using the machine. The OSCAR software over at apneaboard.com does a fine job of revealing each night's sleep record, and mine are always excellent with a running average AHI of 0.6.
Your low oxygen suggests to me that your machine is not set up for you properly. It isn't 'splinting' your airway like it should, so you end up hypoxic. This is not what CPAP is for, to be endured and to offer no remedy. I urge you to register at apneaboard.com, download OSCAR, and if you have an SD card in your machine (you should), remove it and let OSCAR analyse you and your machine's interface. Also, you might wish to place your results before the crowd there and let them offer you some advice or feedback. We do a pretty good job, especially if what you have isn't working well, but it is still the right equipment.

Thank you. This has been going on for 30 years. I started with 1 low dose melatonin and through the years have upped the ante until I don't think any of it is working anymore. I don't want sleeping pills. Just another hook. I'm 73. Not plagued with anxiety. Neuropathy doesn't help. I might try s sleep study? Thank you