I harldy know wherer to start
Well, you can start by telling us what the circumstances were around your affair. Why did you feel you deserved to do something that you knew was wrong? What was going on in your life at that time? Have you cheated before?
You need to lay out all the data - if not to us, to someone or just write it out for yourself. Every relationship. Who ended it. What are your patterns?
Once you have the pattern, you can hypothesize and then refine and repeat. Is it narcissism? Just "normal" selfishness? Etc.