Boyfriends ex messed up his sex drive

I have started seeing my boyfriend a few months ago. And when it has come to us having sex he cant manage it. We have talked about it and he also finds it hard to be intimate. He says that this has happened with everyone since his ex split up with him over 3 years ago. He says he really want me and we get really close to doing it then when he can't he gets really mad with himself. What can I do to help him.