This is the only part of your post I wanted to comment on because I think it pretty much sums up everything about you.

Your at war with yourself. Why ? Because society itself is a contradiction.
Where is this war your fighting coming from ? Societies contradictions or from inside yourself ? Why do you disparage yourself because you are overweight ? Because society says it's not healthy for you or because you can't get into that dress you'd love to wear ?

You say you are a "fat undisciplined piece of trash ". That is a self esteem issue. You can either accept the way you are and be more happy with yourself , or subject yourself to the discipline needed to reach a certain goal.

Where does this mysterious discipline come from anyway ? Is it a learned response ? Is it an innate part of our personality ?

People who want to make money off your misery are the ones that bombard you with all the mixed signals that have jammed up your brain right now.
You live in a world where they try to make everything easier for you but on the other hand much harder.

If you were to living in a different country or society from ours you probably wouldn't have this excruciating pain of having to choose. You would automatically get very thin from starvation. Circumstances and probabilities.
What a great role they play in our lives.

Anyway stop beating yourself up over this and just live your life. If you are hurting anyone it's only yourself. When and if you ever want to change , you will.

BTW: Even if you can get the discipline needed to end you inner quarrel you probably will wind up with a different reason to beat yourself up as most people with mental illness seem to do. Just an observation.

Best to you