The problem is YOU. Like you said, people can tell you that you’re good looking a million times, but it’s not going to get through your head. Negative thinking is a HABIT. It’s not something people are born with, it’s something that they train themselves to do. The longer you think negatively, the harder it becomes to stop, but it’s not impossible. Start by finding five good things about yourself, I don’t care how small they are, I know they exist. :) Write them down and keep them where you can see them (maybe on the mirror?). Next time you go to think “my nose is too big” STOP YOURSELF. Think “I like myself because I have nice skin” or “I like myself because I’m hard working” or “I like myself because my eyes remind me of my mom/dad/etc.” The more you catch yourself in the middle of thinking negatively and correct it to thinking positively, the less often you’ll think negative things in the first place.
I think it matters that you are attracted to the person you are with, but it shouldn’t be the most important part of your relationship, just a plus. I do know that the longer you’re with someone and the more you love them, the more attractive they become to you. It’s good incentive to stay together :)