My work ethic needs serious improvement

I keep telling myself I'm going to knuckle down & work hard but I never do. I'm like yeah I'll do three hours of homework tonight, then I do half an hour. It's so frustrating, I wanna work hard but I cant motivate myself until it's due in the next day. So I've sat here tonight doing shit all, writing the odd history note and I know i have more homework but i'll do it tomorrow. Then I dont. I'll sit up the night before until midnight getting it done, then I panic that it's not good enough. I'll hand it in and make excuses about why it's so shoddy. The teacher gives it back the next day with 'excellent', '10/10' or 'A' scrawled across the top. I'm relieved. I think 'yeh I'll be fine, I'll totally pass these AS levels' and the cycle starts all over again. It's just not good enough, I want to be a doctor, I think. But am I happy? yes. So fuck it, you only live once, if I fail I'll go study cosmetic sciences.