Need motivation for everyday?

I really need some sort of motivation for life..I'm 15 and I'm not like suicidal or anything but really unmotivated...I am often behind with schoolwork or don't work hard at all because I don't have the motivation to be bothered. I find myself terrified of people more popular than me and avoid them, and I have really low self esteem. I used to be a really confident person until about a year ago when I started looking more closely at myself and I saw how plain and chubby I was. I am plagued by cynical and negative thoughts, I am the 'fat friend' in my group of friends, and I believe I don't really have a purpose and they don't really like me; and I let people walk all over me. I'm not really shy and I have a bubble personality underneath but I am afraid no one likes me. Every time my friends achieve things better than me (usual) I want to cry and can't find the motivation to work as I get depressed from it. I keep trying to lose weight but when I get depressed I eat as much sugary food