Well, 6 inches isn't really below average and it seems to service pretty well. My wife and i have a friend couple. The guy has a 10 incher with quite large girth and they barely have a sex life because she can't take it. She's in severe pain after and after a few months of dating, that fun wore off more and more.

There's uh aids/extenders he can wear. Sleeves of sort if you actually miss something larger. Focus on cunnilingus or take it anally - often that can make it feel much larger. Exploring some things like that, or kinks, could repair some bonds here.

You've hit on an insecurity that isn't going to to go away any time soon but it is repairable. Good luck!