How many chances is too many?

Me and my boyfriend love each other very much! We were together for a year and a half before we broke up the first time (he broke up with me) just because we were fighting over little things. Also, during that time i found out he had lied to me about talking to his ex girlfriend behind my back, nothing serious though. We are a long distance couple and it's hard on both of us. After we broke up we went 5 months before we got back together and I gave him a second chance. Things were definitely better and I could tell he wanted to change and really does love me. But he was talking to a girl behind my back for a little while and i just recently found out that they kissed back when we were talking about getting back together. I just don't think I trust him anymore and I was always just a little wary about him keeping things from me and lying about small stuff, which is stupid. He wants me to give him another chance and achance to rebuild that trust but I just don't know if it is worth it to see if i can trust him. It would take a long time to build back that trust and I don't know if i should move on or try again. Thanks for any input! :D