What should I do, I've messed everything up?

I've been with my boyfriend 3 and a half and lived with him for 2 and a half. We've always been sure we're going to be together forever and everything and said that to eachother. Recently I've been feeling bad in the relationship because when I draw his attention to things that are upsetting me he was just dismissing me or mocking me about it.

In the past couple of weeks I've been getting close to this other guy. I don't fancy him but didn't stop him from getting close to me even though he knows I had a boyfriend. The past couple of days I kept thinking about him but I don't even fancy him. I think really I'm just scared about being in the relationship like it is now and the situation made me think about that more.

Anyway, apparently my boyfriend was suspicious about something and read through my texts and found one from a friend alluding to this situation and asked questions about it and I was honest with him. Now he says he doesn't know if we can be together anymore.

In his view I've been cheating on him even though I haven't done anything with this guy. He doesn't know if he can trust me again. And he says he can't love me anymore.

We had a long conversation and when it came up I was asking if he was sure and we were definitely going to break up but he said he doesn't know and he just need to focus on his uni work and decide later. I don't know how to convince him that we can work this out. I'm 100% sure I want to be with him I just was having some problems and maybe this was a cry for help or something. I didn't realise how much this would hurt him and I just want to make everything ok again but have no idea how.

Sorry for how long this is, just trying to give you all the details.