We all compare ourselves to others, but it makes us feel crap. I do it all the time. Among other things, I look at other couples and I say to myself "why aren't me and my boyfriend like that? What am I doing wrong?" or "Wow, that girl is so pretty and gorgeous, why aren't I like that?".

When you get those thoughts, just say to yourself "look, I might not be the smartest or the prettyist person, but I am a good friend, I am reliable etc etc (put in what you're good at). If everyone was like that person, the world would be a very boring place. I bring my own individuality into the world, and that is a good thing"

And also, when you do start thing negatively, change your thought pattern. Think of something happy, something you are looking forward to. It might be something simple like having a nice meal for dinner or something big like the end of exams.