How to feel better about yourself?

I have a brain tumor that has caused something called cyclic cushing’s disease – basically this causes weight gain, ugly deep purple stretch marks, a puffy face, hair loss, etc. I’m 20 years old and I’m not being conceited but I used to be quite a pretty girl. I don’t look like I did a year ago. Now, I’d consider myself horribly ugly. I can’t look in the mirror, I don’t want to go out in public, I don’t want my boyfriend to look at me. It makes me terribly depressed and I’m tired of living like this. There’s not much I can do about my appearance, but how do I make myself feel better? I really need help with this.