In Need Of Big Help

I was so excited to go on duty last night. I even posted a status on FB about my persona of being addicted to my work. I didn't know why but I was really excited to wear my scrubs, have my rounds, and be a nurse for 12 hours! I came on our station like 40 minutes before the handoff time. I received the different pieces of information regarding my patients as early as I was in our workplace. I had my rounds at exactly 6:30 in the evening. I had a great time. But why why why why on this Earth, should I need to stay more than two hours from my span of work. I went home late again for 2 & 1/2 hours (well, it's more acceptable now than my previous check out of more than 4 hours beyond my working hours). It's quite terrible because I can't understand. I was always like this for about 4 months already and I'm starting to feel that the dedication I have is momentously slipping away from me because of too much tiredness and stress. I can't blame my job of being a nurse for experiencing these things. I hate myself. I don't understand why I always stay late, why I need to be OC and paranoid at times. They said that you're incompetent if you can't finish a task within a given time. It's like having no answers for the timed written exam your teacher formulated during your school years. But how? I was always trying my best but I don't fvckin' know. I don't know the problem. I don't understand. Help me.