You're not the first couple to have "crappy sex".

You're not the first couple to have lack of communication.

You want advice about how to stay in this relationship, yes? I personally don't think you should, but I will give you advice based on what you asked:

If you want to keep this relationship from going to shit, you should talk about how this situation was handled (it was poorly handled).

You both admitted that the sex is bad, this is okay, this could be salvageable had it been the stand alone problem. You two could have had open dialogue about this and soldiered on.

That's not what happened though. Your boyfriend, who is supposed to love you, respect you, and be honest with you, went to an ex-fuckbuddy and told her how crappy sex with you was.

That's a breach of your trust. We agree this happened, yes?

Knowing that, I think you need to focus on how your boyfriend can regain your trust. You asked how to get past the feelings of betrayal: that's how. Your boyfriend needs to work on regaining your trust.

Every couple has problems. The moment the relationship starts breaking down is when one person talks about the problems to SOMEONE ELSE while keeping the information from their partner. This is what your boyfriend did.

Lastly. You've breached his trust. You went through his text messages. Just because you found something, it doesn't mean that going through someone's phone was the right thing to do. Work on that.

The above advice aside, perhaps you should include more information about why you want to stay in this relationship. Based on what you've written and based on the brief length of this relationship, most folks here are going to advise you to walk. No hard feelings.