Inscrutable Banana
There isn't much that can be said about loved ones dying. It's always hard despite knowing that it's inevitable, whether the life in question is cut short or has reached its natural end. My condolences on your losses.
As for your parents, I don't want to sound too presumptuous so I'll try not to speak with much certainty on matters I don't fully understand; however, they seem to be actively pursuing a new direction in their respective lives and it's having the unfortunate side-effect of leaving you feeling left by the wayside. Their inability to relate to the types of problems you suffer from and their “pick yourself up by your boot straps” attitude no doubt has them feeling like you'll be able to pull through without them. That being said, by your description some of their behavior is rather cold to say the least. It's disheartening, but if they have failed to understand the severity of your problems thus far and will continue to do so, there isn't much you can do about it. It's hard for people who don't have personal experience with an anxiety disorder to understand how much of a burden it can be, the intangible nature of such issues obscures their ability to see it as a “real” problem—“it's all in your imagination,” after all. The older and more set in their ways people are, the harder it is to change how they perceive something they've already settled their mind on. As hard as it is, you may just have to learn to accept that you aren't going to reach an understanding with your parents on the matter, not now and maybe not ever.
All I have to go on is this thread, but you strike me as a rather intelligent individual and you certainly write better than some of the people I've encountered who have finished high school and continued into college. Although, due to that I suspect that academic performance isn't necessarily where your concern lies. I found grade school unbearable and somehow managed to weasel my way out of a large portion of it and all of high school (turns out that wasn't a great decision, go figure...¬_¬), so unfortunately I can't really offer much in the way of advice on the various pressures encountered in a school environment. And in regards to worrying about how your friends perceive you, well, if they're really your friends it shouldn't matter how much money you have, what food you eat, or what school you can afford. Hardly a revolutionary thought, I know, but it is true. It may be hard to push aside your insecurities, but you know who your real friends are if they're still around despite less than ideal circumstances.
I'm not comfortable saying much about medical matters as I have no real knowledge on the subject nor do I have much personal experience to draw from. I've thus far avoided taking any medications for both personal and financial reasons, but I do know that they can help some people. If you believe there's a real danger of you abusing a medication you should probably only consider pursuing them after you've exhausted all other alternatives. As far as physical symptoms go, I don't really know what to say. Prolonged periods of extreme stress can encourage the development of ulcers, among other things. If you're already experiencing gastrointestinal issues, it might be wise to avoid taking medications and eating foods that are known to be harder the digestive system than others.
You may not be getting the support you need from your parents, and that's highly unfortunate, but you aren't alone. You have your friends, your sister, and your boyfriend to lean on for some support as you try to work on yourself—whether that means through your own methods or with the help of a professional. Try to focus on what you do have rather than what you don't, let the good people you do have around you know that you appreciate them and try to give them as much support in their struggles as they do in yours. Given enough time and effort your life with surely improve in the future.