Why can't I stick to a healthy lifesyle? 5'7 and 250lbs?

I'm a 21 year old 5'7 250lb woman. I know that I'm very overweight. I know what exercise I need to do and what food I need to eat, but I can't do it.

I eat all the time, even if I feel full. I exercise sometimes, but then the next day I'm back to eating and sitting around. When I eat, I know it's bad but I do it anyway.

I work full time and I'm a full time student. I also do volunteer. In other words I'm not home all the time, but when I am, I eat.

I know many people that have eating disorders have emotional trauma in their pasts, but I've had a pretty average life. I wasn't fat until my sophomore year of high school, though I thought I was fat since second grade. Looking back at pictures I see that wasn't true. .

Sorry if this is a lame nonspecific question, but I just feel like I'm running out of options. I really want to be healthy, but I can't figure out why I won't let myself. Please help me if you know why I act this way. I'm desperate.