I hurt my ex boyfriend twice by breaking up with him. We've been talking for a bit and now I like him again but I don't want to be his girlfriend again because I might end up hurting him again I don't know what to do

If you don't want to be his g/f again than u answered your own question. Also, if history is any guide than you will hurt him again unless you have resolved what made you break up with him twice before, which I don't think you have done.

It's also not fair to keep him hanging, I'm in a situation like that and its almost as if the guy just wants an answer, be it good or bad, stringing someone along is not fair.