I've not dealt with OCD or anything, so I'll just tell you what I'd do: study up on horse body language so you know if a horse is going to bite, snd watch a video on correcting a horse who bites. When you go into the area, you'll then know what to do and, while it might be scary, it won't be the world-ender. I used to be terrified of grabbing a horse from a pasture, and around horses in general. I was always afraid I'd hurt them, because I sometimes have anger issues. My trainer showed me that there was nothing I could do with just my hands and feet that would hurt them, and from then on when I walk into the pasture, I think 'Yeah, you could bite me, but guess what? I can PUNCH you!' The horses sense confidence and won't try challanging it. Also, most fears build from a helpless moment. knowing you have recourse should help.
Last bit of advice: don't agonize over what could happen. Focus on other things, like the horse's markings, or hooves or try guessing what it is used for. You don't get on a horse worrying about falling off, do you? No, you walk in and tell yourself that you are not going off, and tough titties if the horse wants you off.