Hoo booy....
Coupla suggestions: The novelty of the CPAP is still affecting you, or it's actually impeding your sleep. I can't help you with your fit or your settings...you'll have to figure those out, but your supplying RT can help. As for the novelty, we often suggest that you simply read at night with the mask on. In bed, or just wear the mask and hose in the living room...the point is to get used to having it in place. As your mind unlearns dwelling on it, as you get calmer, its impact will fade and sleep will come more easily.

Melatonin is a double-edged sword, like most things we label as 'aids'. Your brain ought to, normally, make it in sufficient quantity to help you to sleep naturally. Taking it every night is, based on what I have read, not normally advised. You are better off taking it ever second night, or better yet, every third or fourth night...UNLESS...you have a prescription and it says differently. If you can stand a short anecdote, I had both a pre-screening angiogram and then later a catheter ablation last year. After both procedures, I had a lot of difficulty falling asleep and staying asleep. I asked my GP for help, and he prescribed a very stingy supply of three 7.5 mg tablets of Zopiclone. My pharmacists quietly recommended that I break them in half because he knew about my sleep apnea. So, while the reduced dosage worked, I knew from experience not to take even half a pill on consecutive nights. Instead, I would take a 3 mg melatonin one night, tough it out for two, take the Zopiclone, tough it out again for two nights, and then take a melatonin. It took about three weeks before my mind let go of the whole bother about both procedures (neither was 'bad' or horrible, just a lot of bother and time away from home, a disrupted routine if you will..) at which my normal sleep was re-established.

So, familiarity with your equipment, belief in its efficacy, that it is helping you, and finding ways to calm yourself without relying heavily on medications and soporifics, is the way ahead. I'm not a health professional, and maybe our personalities are markedly different and you know that you need another course. But try not to rely too heavily on medicines unless they are prescribed.