What's wrong with me?

When the tiniest thing happens, I always tend to blame it entirely on me and make it seem like I'm a terrible person, and I suddenly become overwhelm with this really intense guilt, like I don't deserve to be happy or anything. I can't explain it very well, but I want it to stop. I start thinking all these things, and my mind is just chaotic with all these outrageous thoughts. I tend to blow things way out of proportion and make it a big deal when really it shouldn't be, and I also do all this in secret, so I have all these bottled up feelings and it's really annoying. So if I am having fun I start thinking at the back of my head 'I shouldn't be having fun. I don't deserve to.' I just emotionally torture myself for no particular reason. I am 16 years old, and I am a pretty good student. I work hard and I get along really well with my family and have a close group of friends. Could somebody tell me what's wrong with me and how I can stop being like this? Thanks.