My boyfriend told me he hooked up with someone else when we were on a break.

It might be common for few people but Some may mind it. So it differs from person to person, society to society and culture to culture. If you think it's normal then don't let him go because it was during break up and if you don't think its normal for you then you need to take a better decision.

It was his choice to do so, it is never a mistake it is always a choice, but there are other better people out there.

You really need to sit by yourself first and think in your head if this is something you can deal with and manage. If this is a deal breaker for you in terms of continuing a relationship, or if it's something that has turned you off to the idea of continuing romance with this person. And either option is okay. Don't worry about what friends will say, or what others will think. Really sit and think about the choice that calls to YOUR soul the most.

Speak to your boyfriend and ask him what caused him to do that. Maybe he has a strong friendship with that person and they have a connection. Ask your boyfriend if she holds any importance to him and reassure him that you would never do that.

You were in a break and he was honest with you. He had the right to see whomever he wanted, as did you, if you were broken up. I'd be grateful that he was honest with you and move past it. If you can't then your relationship will struggle.

The best thing to is leave him. Yes it will be hard but if he cheated on you while you were break then it's clear he is not worth your time and there is going to be someone out there that will try you right and will enjoy spending their time with you then without you.

That sounds like it might be really difficult for you. Some people interpret "taking a break" different than others - he might have thought that taking a break meant it was okay for him to see other people during that time. The best thing is that he told you, which is better than finding out down the road! Have you tried talking to him more about this?

Give him a chance to explain. Him being honest about it already says a lot about how he might be willing to make it up to you. Or, if he said it out of anger or spite, then he might not be the right one for you. Try talking things out first though. Understand his viewpoint on the matter, share your point of view, and talk things over calmly. If all goes well, then you can both move on from it with a stronger relationship.

No matter what happens, you decide what makes you happy. Technically, you were on a break, so there was no "relationship" unless otherwise stated or agreed. No one else can tell you what to do, so if this makes you sad and not want to be with him, then don't. As long as you don't take it out on him, either.

Everyone needs to set the boundaries of a relationship in advance, to avoid misunderstandings. "On break" means different things to different people. Maybe he understood that there was nothing wrong with him dating others?