Cheating Boyfriend

I have been dating my boy friend for 2.5 years and in the begining of our relationship he would text many girls things like want to come over and cuddle. He said he did this because He missed me and needed someone to cuddle . I decided to give him another chance since we had only been dating a month at the time. Then once again he did similar things like flirting with other girls by text in may of 09 then he actually flirted and lest with a girl at his brothers birthday in sept of 09. Then in april of 2010 we went through a rough patch and he started to text other girls again and I am pretty sure he met up wih them and I asked him who these people were and if it was a girl and he lied to me and said it was a boy when it was really a girl texting him. Now it has been 6 moths since we have had sex and in april I found out he was texting other girls again. I was devistated because he had promised me that he would stop doing this and he broke his promise. So in the end he made me another promise that he would stop doing it but idk if I can ever trust him again and I cant talk to him about it because I feel like it will ruin what ever chance we have left of staying together. Also over the years I have kept track of the condoms he has and slowly they have been "disappearing " and when I ask him about it he says idk what happened to them. And no one else could have used them because he lives with his elderly dad. So I am pretty sure he has had sex with somone other than me.I want to try and save our relationship but I am slowly starting to feel like it was my fault that he cheated on me and I can't trust him any more when I really want to . Is there any advice anyone can give me about how to cope with this and how to help my relationship get back on track?