Hello, I see two problems, and one really good thing.

Really good thing - you know it's all in your head. Knowing this and being aware of it is what's going to stop you from letting it affect you or the relationships too much. You're in exactly the right place to be able to deal with this

Problems:
1) You are projecting your own feelings. You know you cheated, so why wouldn't someone else cheat on you? It sounds like this is what's going on in your mind. I've been the same in a relationship before where I knew I wasn't completely into it and I knew I compared my boyfriend negatively against my ex and felt very disappointed in what I had, so I assumed he was doing the same. But it's all in your head. Just because you cheated, doesn't mean someone will cheat on you.

2) You are comparing yourself badly with others. This is the biggest problem. Learn to see yourself as equal to others and as valuable in your own right. The easiest way to do this is to recognise that it is simply a fact you are 'good enough', accept that you will still (erroneously) think you're not good enough, then notice those thoughts when they come up and remind yourself they are just thoughts and are not reality. Self-esteem, unfortunately, is nigh impossible to change as it's very deeply embedded. Instead of trying to increase my self esteem, I have found it more useful to accept that I do have low self esteem and that it will affect me somewhat (but hey, no one's perfect and we all have our crosses etc). Just accepting that you have low self esteem and that you will be disturbed by thoughts that do not reflect reality can be enormously helpful, because you learn to discern between your thoughts. If you have a thought that compares you with another or says something negative about you, you can learn to recognise that it is just a thought and not to be listened to. You stop letting it affect you, is what I mean. Although CBT professionals would disagree with me here, I would encourage you not to try and fight the thoughts - i.e. if you think you're no good, don't start trying to prove to yourself that you are (even through a dialogue in your head). This only makes things worse. Don't challenge the thoughts and anxiety, just accept they are there and that you can cope just fine anyway.

Hope this helps.