What should I do in this situation?

my boyfriend and I broke up for a while but then we got back together and we have been together a month now again. We were doing great but then tonight he brings up something from the past and that was sometimes he wants to be single. And after he told me that he started to talk to me all defensively. Now to back up a little earlier I saw him texting a girl that in our past relationship caused a lot of problems. But I'm trying to look at our new relationship as a fresh start and trying not to let the old stuff get to me. But see I don't understand why every time this other girl comes around he always talks about being single. I would talk to him more about it but to be honest I'm scared because whenever we talk about something like that he tells me that i'm always putting him down and that he doesn't think that we will work out and that he only started dating me again because he felt bad for me but when I started to get mad about that he said he just said that because he was in the heat of the moment. But on the other hand he saved up to buy me a ring and I love him so much and I believe he loves me too its just that hes young and doesn't know what he wants yet..any help?