How do you deal with your insecure SO

This is a question for those who have an insecure SO. If you don’t, cool, breaking up with my fiancé isn’t an option though. How do you deal with someone who has insecurities from a previous relationship and/or childhood? It’s just so annoying reassuring him all of the time, or feeling like I have to explain myself/defend myself all of the time. He isn’t controlling nor does he tell me what to do, he is just... annoying sometimes. Help?