I feel a lot like that myself right now Gencat. Know what I tell myself? "Too damn bad! You're living anyway!"
When I'm feeling horribly low, I tend to think that the best self-punishment I can have is to keep myself alive. So I do, and eventually the feeling passes and then I realize how utterly silly it was. I also daydream of ways to die that I wouldn't have any control over, and thus I couldn't act on it.
I don't think that you're worthless or a failure. I will spare you the chance to argue with me on your body as I don't know what you look like (because no matter what I could say right now, I know that you would discount it!). But I can say for a fact that you're not a failure because who really decides failure? We're born into a rather stupid society. Think back hundreds and hundreds of years - you know how to read and write! You'd have been SOOOOOOOO AWESOME back in medieval times, right?! Also in those days, if you WERE in fact larger, you'd be admired for being able to get that much food!
We just unfortunately live in a world that really has a lot of expectations that are totally made up. It's those expectations that are worthless, NOT YOU!

I think that you're worth a lot. You're worth staying alive for, or how else will you get to see the future when we meet all the aliens?!?! ((Ok... just trying to see if I could get you to crack a tiiiinnny smirk there). But seriously. Aliens. Must stay alive to see if we get to find any. I think THAT is worth living for, right?! Wouldn't it be cool to be the first generation of humans to encounter alien life?! Even if they're just discovered and we never MEET meet them. You know?