I met my ex in my freshman year of college. i rejected him at first but then in March a few years ago i decided to go out with him. our relationship lasted over a year. we broke up because it was very unhealthy and there was no trust. after we broke up, we hooked up at random times but ended it because people found out or he did the same nasty things again so i couldn't deal with it.
Recently, this school year, he contacted me and we decided to get together. he had a girlfriend but we still had the subtle coffee conversation at starbucks. after that we hung out a few more times until he invited me over at his girlfriend's apartment (where he was staying). we ended up having sex... in her bed, and it was honestly 50% amazing, 50% WTF. We decided to keep doing it and we did it several other times in her bed again, at my apartment, and his apartment. At first, it was just casual by not kissing goodbye or holding hands. He was still with his girlfriend when he kept on lying and bailing on me for his friends or her. I told him I didn't mind him being with her but he didn't have to lie since we weren't together. Gradually after a few months, we started kissing each other goodbye, holding hands, cuddling, and hanging out longer after having our rendezvous. Also, we have hung out at bars a few times with him and his friends. he has texted me from every couple days to every day now. Now they are not just about sexual stuff or meeting up to hook up. Before I left for break, we had lunch before we did it again. When I was back on campus and we were both in town, he even invited me over to his apartment to swim. He has texted me nearly everyday over winter break, with 2 days being the longest without texting and he has surprisingly not been texting dirty or asking for sex.
The thing is that I realize this is wrong and I am planning on tell his girlfriend. They are at the stage of moving some of her stuff into his place and she STILL has no idea that half of her relationship has been a lie. The thing is that I want some of your suggestions on how I should tell her. Thanks