Obesity and depression go hand in hand. You eat because you feel unworthy and unlovable. You lay in bed with the covers over your head and mourn the fact that you can't exercise, all while, miraculously, surprise surprise, NOT exercising. You look at yourself in the mirror, or on a scale, and think you're a failure. So you're unlovable and unworthy and so you grab the ice cream and get into bed and think about how no one understands you and how awful it is and oh poor you and look the weather stinks and those grey skies are an omen, they are telling you that you'll never get there, you'll never get it right, and it's all for naught and we're all going to that great vale of tears anyway so why not have another piece of pie? And the world agrees with you, and tells you you're worthless in all sorts of subtle and not so subtle ways. And, God help you, you believe it.
Lots of people feel/felt unloved, unwanted, undervalued, and all the other un's... they didn't all gain weight, though. Some turned to other forms of self-hatred, like smoking, crime, drugs, prostitution, alcholism. And some didn't engage the self-hatred at all. What made them react that way? That's a good question. Why didn't they succumb to the self-hatred motif?
Why should it be up to others to support your self-esteem? It's not their business and not their job. They have their own problems - you just can't see them as well as they can see yours.
As for the world telling you you're worthless, well, most of the world is in the same boat, just a different kind of boat.
Not since about grade 10 have I seen a large person treated derogatorily. I've worked with them (yes, large people are employed everywhere), had dinner with them, and so on. They go to school, they have friends, they take holidays, they have money... I don't see the discrimination. Perhaps it's something else.
You might dismiss my observations since I'm not, nor ever have been, large, but I just don't see the prejudice. Not saying it doesn't exist, but I don't see it.
Perhaps some of it is your sensitivity to it - you may feel it even though it isn't there. Is that possible?
There are all sorts of people out there with worse problems than being overweight and some of those can't do a thing about their problem. Like Shewolf said, it's a choice we make, and when you have a choice, it's up to you. Nobody else is responsible for how you feel about yourself.
I know this sounds harsh and I'm sorry. But I'm really not responsible for how anyone feels except myself.