Remember that skinny does not equal beautiful. Remember that skinny does not equal worthwhile. Your A is not beautiful because she is skinny, she is beautiful because she is a loving person with qualities that you admire. If you find her attractive, I'm willing to bet that you don't sit around saying, "I really like how skinny A is. It's her best feature." I know that when I look at my girlfriend, I think about how much I like her hair, or how lovely her eyes look, or how nice she looks in that sweater. Of course she thinks that she's not hot because she's not anorexic like she was when she was young, but I love her, my husband loves her, and her husband loves her. Do I think she's more attractive than I am? Of course. But like GS said, it's not a competition. I would like to suggest that you keep a self-esteem list--write down everything you think is a compliment and everything you like about yourself. Make sure you write down every little thing, including if you're kind or if you're loving or if you just think you look cute in a certain outfit. It does help. Spend time every day thinking about what's on that list or what you value about yourself.
I do differ from GS on the point of worrying about becoming a burden. Of course you don't want to be a burden, but keep in mind that these people love you and care about how you feel. They don't mind reassuring you that they love you and find you attractive. (BTW, did you know that most men find women who are pregnant with their child incredibly beautiful?) They don't want you to be in pain. Be self-aware, and when you need to ask for help from someone who loves you, do it. But be self-confident also--you are a strong woman and you can trust yourself.
Also, cut yourself some slack. Your hormones are waaaay out of whack right now, and it won't get better for a while. I live with an endocrine disorder which causes hormonal imbalance, so I have a few tips. When you feel yourself in "that place" where you're feeling like you might be getting too emotional, don't be afraid to take a moment to yourself. Take a breather and do something nice for yourself, like a bubble bath or a little bit to lie down and read a book. Problems will still be there when you come back, so you can take a break and come back at it when you've had a chance to recharge. If you can't stop thinking about the problems, write them down and walk away. Writing them down helps you feel like you won't forget if you stop thinking about them. Practice taking a deep breath and letting go of the things you can't change or have no control over.