A middle child then. No surprise. Consider this, and I'll leave you with this because these are things you need to work through in therapy: You naturally competed with your older sister when you were born. You looked at her as a role model because that's what younger siblings intuitively do. But then you were pushed off the youngest child throne by the arrival of your younger sister, which is the natural experience of older siblings.

So you're eternally caught between being intellectually insecure because of your older sister, and you're insecure regarding your appeal to other people because of your younger sister. You naturally, as all children in adverse emotional circumstances do, blamed yourself for this experience of abandonment. And you blamed it on your looks in particular because your mom would always emphasize the importance of your looks by telling you that you're pretty.

You are pretty (not that it matters in terms of your happiness), but on some deeply buried level you always feel as though you weren't pretty enough because otherwise why would your parents have "abandoned" you in favor of your newborn younger sister?

I put "abandoned" in quotation marks, because it isn't real abandonment, but a child's soul is a fragile thing, and it doesn't take much more than the sensation of abandonment caused by the arrival of a younger sibling who needs all the parental attention to hurt a child's soul forever.

The looks part became particularly important because your younger sibling is also a girl, and because your mom apparently emphasized the importance of your looks. That's as far as I can go here. But just the fact that I correctly guessed that you're probably a middle child should clue you in on the fact that your entire problem has absolutely nothing to do with your looks, and everything with your life history.

Go get therapy. Get better, feel better, live your life. And forget all about those silly plastic surgery ideas, at least until you've been in therapy and talked through all of those issues with a person you feel secure and accepted with. Good luck.