I was in the same situation as you about 12 years ago. I wanted to see everyone but I could only think about the fact that I had gained weight (I was by far the skinniest kid in my school). So I declined. And a couple of days later I heard from EVERYONE about how much fun it was, that I was missed, that they saw so and so etc. And BOY - did I regret it. I failed to notice that I was very successful in most aspects of my life - I just saw the one thing I was not proud of.
I had the opportunity again about 5 years ago at our schools 50th. I did not miss out this time. Despite the fact I was now divorced and things were not as peachy as they had been before. I WENT! And I LOVED it. And people were all different. Some were wealthy, some were not, some were healthy, some were not. I saw my Jr High Crush (who made me feel great), I spent time talking to Ex's cousin (who died 7 months later unexpectedly) and I renewed many friendships and I stay in touch with them on FaceBook.
For me - there is only one piece of advice to give you.
