I had sex at a party...

So my so is away at college and were on a break rn but we still talk all the time and i love her more than anything. She really hates parties but i went to one last night. I ended up kissing on of my best friends and taking my exs virginity. I feel really really really bad about it. And i hate myself rn. Please dont give me any crap about cheating on her or anything itll just make me cry. Also some background info. One of the main reasons were on a break rn is because she cheated on me for about four months and i dont trust her anymore. Ive been having doubts about us for a little while cuz im so afraid shes gonna do it again but after last night i know shes the one for me absolutely no matter what shit happens. No matter what she does. I love her so much. I just dont know if i should tell her what happened or not since were on a break. I dont wanna hurt her. And if i should tell her what should i say how do i go about it. Ive never been in a situation like this. Please help me