how to improve my self esteem

My self esteem changes from day to day. Some days, I think I'm beautiful. Other days, I think I'm the fattest person ever. My friends and boyfriend say that I'm not fat at all, but I can't shake it from my head that I am. I've tried to go on diets and work out, but they always seem to go overboard to the point that I skip meals and only eat the minimum I can each day. I've tried to download a diet tracker to try to keep myself motivated to eat meals, but that just made it worse. What should I do?