I'm sorry you're suffering so much. This sounds like it's been going on for a while. I think you deserve some relief. I think seeing a therapist is one way to do this. I don't know that you need to see anyone who specifically advertises as a 'sex therapist,' but in your initial phone call/consult, you should probably ask if they feel comfortable/qualified working with issues of body image, sexuality, and relationships. Most are.

Many clients find that just expressing their issues and feeling understood helps them feel better right away. A good therapist will not minimize or invalidate your concerns--that is, no good therapist will say 'size doesn't matter' and call it a day. Instead, they should help you find ways to either accept or change the things about yourself that cause you pain.

Good luck with everything and hang in there.