Cheating rant

So this is mainly a rant, but feel free to chime in advice if you have any. Me and my boyfriend started dating last October. Well back in december he cheated on me. Not only with someone who he didnt know, but with a girl who I work next door to and have to see every day and who only came on to him to be mean to me. We broke up for about a month and the whole time he was begging for me back so I finally took him back. i know I may sound like an idiot for saying this, but I truly believe he changed and wouldnt cheat again. I also have to confess that I cheated on him as well, which he still doesnt know about. Ever since we got back together he has been amazing to me, but I have a problem with letting our past and my trust issues get in the way of us. Sometimes I am so mean to him and I feel bad about it but I just see her and think about everything and it puts me in such a bad mood. Idk what to do, I love him but I honestly dont know if I will ever be able to let go of the past. He is planning on proposing to me soon and he's already been married once and Im not going to put him through that again unless i know it will be forever. Let me also add that him and his ex got divorced because she cheated on him so he knew how bad cheating hurts, and he still did it to me.