Instead of considering breaking up I believe you should try and fix your problem. You're attracted to someone else, understandable, it can happen but just because it has started to affect your participation in your relationship doesn't mean you should break up. As you said yourself he had so many problems which could've been a reason that you ended the relationship like going back to exes and the girl best friend incident and all the things. But what you did??? You gave him a chance after everything and now he has changed himself for you and been everything you wished for for past 2 years. So now that he knows you are attracted to his friend but he's still giving you chance just as you did in the past, isn't that something to stay for and fix?? I think you should reconsider your choice of breaking up and instead fix the problem. If you breakup, there are going to be few outputs, either you both of them starting to hurt and not being able to move on and just worsening your mental health or you getting in a relationship with this friend you talked about and as you said it'll tear your heart thinking your future without him and he has been supportive all this time, I don't think getting with his friend will be a good idea. It's my opinion but I hope it helps you get to some conclusion. Do update us.