*Sigh* I know all too well what you are talking about. It's not even relevant what you weigh, you still feel this crushing sense of self loathing and not being good enough. It's something I have struggled with for a very long time (still do!) and it's true that there are probably hundreds of girls who look at YOU and say "god I wish I was that size/pretty" so there is no rhyme or reason to it, it's nonsense- you need to figure out how to stop those tapes (that's what I call them) that constantly run in your mind: I have ugly knees, she is skinnier, I am fat blah blah. When those thoughts enter your mind just tell them to shut up! Or whatever works for you.
Also this sounds silly but I read this technique in an article once and it feels weird but is actually effective: look at yourself in the mirror and tell yourself over and over again "I love you just the way you are", even if you don't believe it, just say it.
I also don't know how old you are...I am 23 and I think a lot of it will ease with age as we gain confidence and wisdom to know that external beauty is so incredibly fragile and temporary, there are so many more important traits a person can have. I hope this helps, love yourself woman!!