What should I do about my friend?

[Helen]

Your friend probably IS self-conscious.
After all, confident people who are happy with themselves don't feel the need to seek attention by fishing for compliments and walking round in revealing clothing.
It's understandable you're frustrated though. There's only so many times you can reassure someone. It isn't your job to cater to her cries for attention. She needs to realise that it's not all about her. Everyone has stuff going on, and just maybe, her friends would like it if she was there for them now and again.
You have two ways to play this. 1)You can all confront her together as a group - tell her how you feel, and that you think the friendship is unequal, and that she acts out for attention. Explain that you are sick of her bragging, and constantly having to tell her how pretty she is.
2) Ignore her when she starts up, and don't take the bait. Change the subject when she starts bragging, and when she calls herself ugly, don't contradict her. Instead tell her it's sad she feels that way about herself. When she's not getting a reaction, she'll be more likely to stop.