I can't stick with a workout plan?

So I'm trying to loose some weight. I'm a 17 year old girl and I weigh a little less than 250. I know it's bad. But I'm really trying to loose weight. And really eating healthy isn't that hard for me but I cannot for the life of me stick with a workout plan. Like I walk occasionally on a treadmill for about a half our, but it's usually like 3 days a week if I'm lucky. I've just started this like a couple weeks ago, and my mom is finally going to get some healthy food tomorrow. I'm just so depressed lately because I feel like I'll never reach my goals, because I can't seem to find motivation to work out. I'm motivated at like 2 in the morning and I like plan out this extravagant workout plan and eating plan and then I wake up and eat a doughnut. Is there anything I should be doing different?