I agree with brokenentity, definitely. And something you said, "The reason I see myself so darkly is because all my life I have been told repeatedly that I'm fat and ugly and stupid and other bad things; by people my age, hell, even by adults, though mostly by society in general." This is huge. This isn't right, and you don't deserve that at all.
The world does become less cruel after high school, HS is always the worst, so I still feel it's important to stick it out as sometimes that's all we can do to survive. But on top of that, it's important to not take on what others say about us. I know that's much easier said than done, but you're human, if there's one thing humans are good at, it's survival. And the part of you that needs to survive knows that you can't possibly be quite as bad as you feel, or else you wouldn't be here on this forum looking for support.
Trust that instinct. You've been fed horrible lies your whole life about your supposed inadequacy. Our society does hold certain kinds of beauty up on a pedestal, gleaming and golden, for all women and men to know what they can never attain... but it's simply false that that's the greater beauty. You are unique, rarer than any jewel in the world, and far more beautiful because of that. I know you don't feel that way, but that is the real truth as opposed to the lies you've been confronted with daily. I know it's hard, seems impossible even, but hold on to that slight glimmer of truth that lies beneath rather than the layers and layers of sludge society has thrown onto you (if you wee to be compared to a jewel

).
Someone else brought up loving ourselves on this forum, not knowing how to do such a thing. That poster, I believe, eventually commented that they had determined (after receiving our support) that loving themselves must start with simply taking care of themselves. Not because they feel they are wonderful or beautiful or special, but because they
know that they must. Don't simply follow your feelings, because those have been taught to lie to you the same way everyone else has. And trust the support you receive here.