Who is the person, or people, who is/are giving you so much s*** about your appearance? From what you've said, their hypercritical attitude kind of makes them look like the one(s) with the problem. If it's just one person, you might do some assessing to try and decide if this is someone you really need to have around you. If it's several people, then maybe a kinder group to hang around with would be better for you. It can be really hard find a new group of people to associate with (I know), but if the people you're spending time around are dragging you down on a consistent basis, is it really worth it?
If it's family that are criticizing you so much, that can sometimes be a little more complicated, if for example you're living with them and don't have the resources to move out right now. In the case of family, maybe an honest talk with them about how they're making you feel bad with their criticism could help.
As far as your appearance, you might take an honest look at yourself and decide if you're happy with your appearance. If not, there a lots of ways you could get ideas about how to dress, groom yourself, etc., that you may be happier with (you could look at images online, notice things other people wear that you like, read articles about "improving" your appearance, etc.) If you're fine with the way you look, though, that's really all that matters (unless you're applying for a job or something, and that's just a short-term situation).