I feel you. I have both OCD and ADHD.

I personally believe that the OCD is compensatory for the ADHD. Like subconsciously, I know that I might miss something because of the ADHD scattered-ness. So the OCD kicks in. And then it's a real matter of OCD and no longer a slight issue of ADHD forgetfulness.

Either way, if you want to go the non-medication route, the most effective therapy would be exposure therapy. That's usually done in conjunction with medication but it can be done without. You just need to make sure that you find a therapist who is experienced in CBT, specifically EX/RP for OCD (I know that's a lot of acronyms). But it's a specific type of behavior therapy that not all therapists have training in.