My Ex is cheating on her new guy with me. what should i do?

Ok so like 6 mouths ago I broke up with her but we were still together like sleeping with each other, we were always arguing after we broke up cause she wanted me to get back with her but i keeped telling wait till summer starts up so i dont have the stress of school when im starting a relationship. During the time that we werent dating i didnt hook up with any girls but a couple tried and i pushed them off I even told one of my friends i wasnt going to prom with her cause i still loved my ex and my ex knows i said that. we still tell each other we love each other. about a mouth ago she met this guy at work and started hanging out with him. i didnt like it but i didnt make a big deal out of it. 2 nights ago she was talking to both of us over skype in a 3way conversation and we told her to pick. and when we did that she said i dont want you to hurt me so im going to pick him. at this point my heart completly shattered. I've been telling her how much i love her so much and how i would never hurt her again for the past 2 mouths but she still cant seem to trust me. The next morning after she picked him we had plans for breakfast (made before she picked him) but I told her theres no point but she insisted that we hung out. we hung out from 10 to 10 all day and did multiple things it started a little awkward but then our natural attraction took over and we went back to how we always were. we made out pretty much all day and were being very sexual when i told her i was getting my yin yang tattoo covered up cause i thought it could represent me and her she told me let it push me to try harder to get her. after that we went to her house and we almost had sex but she was on her period. the thing is i said you should probably tell him whats going on but she said she wont, but i want to be with her so bad and she cheats on him with me should i just tell him and avoid more heart break for him and so i can get my girl back. i could honestly say i could see myself spending the rest of my life with this girl.