Sorry you had to go through this rostfrei
Breaking up was the right thing to do, considering the circumstances.
Next time. As you are getting closer, and making plans to move in and all. You might want to let the next guy know how important monogamy is to you. And ask him what he thinks about the issue too.
If he doesn’t believe in monogamy, it’s better to lose him sooner, than assume he’s feels the same way. Then you wont have find out later that he thinks a little on the side is OK.
You sound pretty sure of yourself, what you want, and what you will and won’t accept, rostfrei. Good for you - those are great characteristics.
Eric,
Here’s the thing about cheaters: They are going to do it again. Ditch them and get away ASAP. It is hard to leave an SO who has cheated, but I consider that the biggest affront that I could possibly find. My girlfriend has told me that if I cheated on her she would forgive me, but I have made no bones about the fact that she will probably not ever hear from me again if I find out that she has cheated on me.
It’s tough, but necessary for your own self-respect.
–==Mike==–
rostfrei,
I’m sorry about what you’re going through. 
It’s an awful, awful feeling. I’m glad to hear you’re not planning to go back to him. I have a word (OK, more than one word) of caution for you: Don’t weaken! One time I said that to myself too – that I wasn’t going to get back together with someone who hurt me. Then, I sort of forgot how bad it hurt at the time, I really started to miss him, he kept on pestering me, I believed it would never happen again, and he finally wore me down. DON’T let that happen to you – if you find yourself wavering, look at this thread again, or write down right now all the ways you felt (sick to your stomach, etc., etc.). You say you’re not going to have any contact right now – that’s exactly the right thing to do. Don’t take his calls if he calls – because if you do, you open the door to the possibility that he will convince you to come back. Remind yourself that if you always run screaming from people who even exhibit signs that they might end up treating you poorly, you will never have to be treated poorly again. I’m not saying there were signs or weren’t with Jason. All I’m saying is that since the day that I realized that I, and my heart, were precious things, and they would be treated kindly and carefully, and cherished, or not treated at all, we (my heart and I, that is) have been treated kindly and carefully, and cherished. It is not always easy (to run away from someone you really really dig at the first red flag), but it’s a simple formula and it has worked for me.