Hi and welcome
My partner struggled with exactly this for years. It pained me to see his struggle, seeing himself as somehow abnormal or damaged, so I set about finding out more and this is where I ended up as well.
It is a good place. It has helped me to feel that our relationship is normal and OK, when the world is often very quick to judge and harsh in its judgements. He is now feeling much better about himself and it seems that makes him able to love me more.
If this is something that is coming up for you in quite an emotional way during your engagement I think you should deal with it before you get married. If it didn't matter and was just a passing whim I doubt whether it would be having the emotional impact on you that it seems to be having. My partner lived 10 years in a marriage that became very painful, partly as a result of this.