What's in it for you? I'm failing to see the upside here. Your boyfriend teases you in hurtful ways, has lied to you in the past, accuses you of lying about a variety of things, doesn't stand up for you to his other partner, refuses to allow the thought of letting you have another partner and uses you to get his homework done. I really don't see any redeeming qualities in this relationship.
You also say that you don't want to talk to a therapist about this because you're afraid they'll convince you to leave him. If the inking of a thought of leaving him wasn't already there this wouldn't even be on your radar as a concern.
165lbs ISN'T fat. It's curvy and womanly. I know there are a lot of women on this board who consider that their ideal body weight that they are striving towards. Thinner and prettier doesn't mean happier by any stretch of the imagination. Anyone who would want you more if you were thinner just isn't worth having you the way you are.
This relationship seems to be making you miserable. You are the only one who can change it. It's scary and hard to leave something you're used to but until you do, how can you know what else is out there and possible for you?
Everything will be ok in the end. If it's not ok it's not the end.