It's great that the neurologist didn't find an organic problem - that means you should be able to get this under control.
One thing to remember - do EVERYTHING in moderation - sit a little, move a little, exercise a little, walk a little - and gradually add as your body tolerates. Building strength and endurance will take time.

Since you asked about activity, I will tell you that, when I was in chronic intense pain last year from arthritis, the pain doc's first recommendation was PT. The last thing my poor body wanted to do was to move! But she was exactly correct, and I saw a PT for many weeks. I now have a ready list of exercises and stretches I do daily, and some I can add when specific aches and pains crop up. I also walk or do physical labor every day. Maybe you can get a referral for PT from the neurologist or your primary provider to break the cycle of pain.

There are a number of exercises shown online for a painful SI joint. Here is one resource showing some of them: https://www.spine-health.com/wellness/exercise/strengthening-exercises-sacroiliac-joint-pain-relief

Finally, there a a couple of home remedies that can sometimes help with acute pain -
Move - if you have been sitting, get up and walk, bend, twist for at least 5 minutes - even when it hurts.
Lie on your back, knees bent, ice pack on most painful area, for 20-30 minutes. Repeat as needed. Larger gel packs or a two pound bag of frozen peas or corn work - refreeze & reuse.

Remember, it took 3 years to get to this level of pain, so it will take some time and effort to reduce. Do everything in moderation - when you are sitting, get up and move a few minutes every hour or even half hour. When you are walking, take a break every few minutes - even leaning against a wall or tree can help.

Good luck,
Sue