You are not fat. You are hot-a-licious. and sexy. Although situps are good, you should do something for your whole body. I know you have EDS, can you yoga? Don't overdo it, pace yourself, you don't need a bulging 6-pack with tiny arms and legs. :-) Also remember. Don't panic. It's a principle that works equally for self-image or interstellar travel.
Unfortunately some people don't look at others on the internet as "people". Other people are just words on a screen, or a picture to them. It sucks that empathy has given way to assholery.