I don't get it - she's had issues in the MR with sex and not wanting it or being able to have it, and has given you grief about you wanting to have sex, but now is having sex with other people????
I concur with the view that you should snoop to get intel and once you have intel, back off and stop. I think you have your intel. Now stop. As you can tell, this is going to harm you and set you back in your own healing.
If you feel like you can't hold it together when you see her to do the taxes, why don't you postpone it a week or two. This way it gives you some space and time to deal with what you just found out and ground yourself so you can get through an interaction with her.
Alternately, do you HAVE to do the taxes in person together? Is there a way around that so you don't have to see her?
Listen, now is the time for super focused self-care. You have to do things that will help you jumpstart your path of healing. Do you have a counselor you can see? I think that putting your focus on climbing and expanding your social network are really good things. Why don't you start expanding your social network at the climbing gym? how long have you been going there?
I am sorry that you are going through this with the new intel. But at least you know now.
