Should I believe him when he says that?

Me and my boyfriend have been together for a little less then a month now. We have dated once before but it only lasted a month at least as well. I care about this guy quite a bit but we occasionally have our fights. The last one just happened to be over my religion. That's beside the point though. The last time we dated he left me for another girl and i forgave him after her apologized profusely. I realize that was probably a bad move on my part but i couldn't be happier. He tells me he loves me and wants to be with me forever. When ever he tells me this I can't help but to think of when he left me for that other girl. He promises to never leave me and once again my mind automatically seems to think of that. He truly is a wonderful boyfriend but i guess what i want to ask is should i believe all the things he says to me about loving me and never leaving me?