Is it normal to say i love you and not meant it?

Ok so yesterday I found out that my bf had 2 apps in his phone that hides things, one is for porno(wish is not an issue, but the other one was for calls and text messages. In this txt he was talking to 2 girls, one of them is a girl he met in the university in august, and since then I've noticed some changes in his behavior with me, and confronted him about it but he told me I was imagining things, but the feeling never left me. He introduce her to me once, she was really cold, looking at me in a really "i don't like u" way, and by that & other things I knew she was into him, but once again he told me it was all in my head but I know him and well, I know girls, so I told him that I didn't felt comfortable with him talking to her that if he please could take some distance from her, he agreed (not really, since a few days after I found out he had erased a call that he made to her or as he tells me a friend made from his cell to her, since then the lies never stop). The thing is that in my ignorance I thought they were just friends, but in the txt I found yesterday he has been talking to her secretly for about a month or so, about seeing her in college, adding her in fb with a bunch of people so I won't doubt, even telling her that he loves her (in Puerto Rico we have te quiero & te amo, I can never understand how you guys know which one is which)she even asked him a few times if I was going with him to the university, the last few txt were more distant in on his part, she kept telling him that he was ignoring him, and asking him if he got tired already of her and he just told him that she could always count with him but he doesn't want more problems. I went to the uni and gave him his cell and walked away, he begged me to let him explain so I did, we have been fighting for few month, and well we had some issues, but he never could explain to me why he did it, he told me that nothing happened between them and we talked with the girl personally she said nothing happened, I'm still not sure. He says he love me that he doesn't want to lose me, that he'll do anything (we were already talking about getting engaged, the day before this happened we were ring shopping) after all that happened I told him that this was he's way out so he should take it, he say no, that he would do anything to prove to me that he loves me and that he made a big mistake but that it's over and it will not happened again (of course I don't believe him) but (call me what u want) I do want to believe him. I told him that he will have to start over with me and prove that he means what he says, but that I couldn't promise that I will stay, because regaining trust is an almost impossible thing to do. I don't have many people to talk to so I would appreciate any advice, real and serious ones. BTW we are both 23, and have been together for 1yr 9mo. (this is not a game for me, nor a child thing, I'm very serious about what I want for my future and he knows it, at least he knows exactly how serious I am now)