If you think you let people down by doing those things, you'd let people down 1000+ more times if you ended your life.. Your friends and family would be heart broken.. Weither you feel like it or not, they would be.. And people you don't even know would be effected by it too.. They'd feel guilty, and alot would feel like they where partly to blame.. They'll spend forever thinking "What if..." "I should of done/said..." "I shouldn't of done/said.." And that goes on for a very long time.. Its just not worth it.. It may even leave people feeling as bad (or worse) as you feel now..
I don't know what to say about how you feel about being 'fat' but then again I know not much I say will change how you feel. Just try and think of it as, thoughts are thoughts.. Just because they are there does NOT mean there true.. And also, not everyone sees you the way you see yourself..
If you feel bad about lying to people, maybe you could try telling your close friends or something, but make sure you tell them exactly how you feel about it.. I know it wouldn't be easier, but atleas they'd be able to understand how hard things are for you..
You only FEEL like your going to fail, and maybe because your thinking that so much, you will.. Believe in yourself a bit more or even just tell yourself 'Yeah maybe I just might do ok..' If your putting the work in I don't see why not, your still trying your best and thats all that matters..
Don't give up, There is a chance that things will get better, your better to try for your whole life, because atleast you can ALWAYS say you tried your best and thats all that matters..