(((mednurse)))
I have experienced the same thing with a few of my illnesses. Medications will work for a few years, and then things get too rough again & another change is in order. It is very frustrating!
In my opinion, it isn't anything that we're doing wrong, per say. But, part of the problem is physical. Fixing these major problems that we've held for YEARS don't go away that quickly. (I know that you've been working on the OCD for 10 years ~ but the other illnesses were untreated until just a few years ago.) In the grand scheme of life, a few years of treatment doesn't erase our misery.
Not that our misery will continue to follow us throughout our lives (at least I hope not!), but highs and lows will certainly be with us in life. Maybe you can try expressing these dark emotions that continue to haunt you through art (music, drawing, painting, writing, dance, etc.) and exercise. Something worth thinking about anyway.
Gentle hugs to you...