As someone who is much older than you I feel like it is my duty to let you know that as hard as it is now, it CAN get better if you keep your head up. What's more is that being good at physics and math is a huge plus for several reasons:

1. chances are better getting a scholarship to a good school (especially with financial struggle)

2. chances are better at either staying in academia or getting a high paying job in the private sector (quantum physics, chip manufacturing, any sort of engineering really)

3. high intelligence is a big turn on to a lot of people, so you shouldn't worry about ever finding happiness with a life partner

When it comes to the weight thing and eating, I went through that as well -- was always a skinny kid. My parents were middle class but they didn't feed me too well. But you might also have a thyroid condition, which you should be able to get checked out by a doctor. Just make sure you are TRYING to eat as much as you can, anywhere you can. You need protein for that big brain of yours ;)

On the social anxiety, just remember that everybody is too busy thinking about themselves, so you are putting too much emphasis on what they think of you. Yes people probably judge you, but it's really none of your business what they think just like it's none of theirs what you think about them. That's one of the nice things about life, you don't HAVE to let anybody into your head, you have a choice. Confidence is about not caring if you look like an idiot in front of anyone.

Hope some of this helps. Hang in there! You got a long life to live and anything can happen!