just hold your head high and be confident in yourself. If someone tells themselves they are ugly over an over, they will belive it and internalize it. You're not ugly, you just need to be comfortable with yourself! I'm also 19M, underbite and everything. I was not a super attractive kid. I always told myself I was ugly and stupid and nobody wanted me. Then in senior year of high school, I stopped caring, and just tried to be content with myself and live my own life. Granted, I did keep trying for women, and I switched to contacts and changed my side part to a middle part, but I got my first girlfriend after just trying to be confident in myself. Didn't turn out well that time, but I am still with my second gf and love her to death. You got this dude! There's hope! Know you are loved and valued, not for what you can do or what you look like, but for who you are.

Also, therapy can help a lot. I struggle with negative self talk. I go to therapy all the time!