My mind is tangled up! Need to Focus! Please Help!?

Ever since the beginning of this summer my mind has been pretty messed up. I currently attend summer school which starts early in the morning and have a part time job which doesn't end until around 9:00 p.m. So as you can well imagine I'm always busy and as tired as hell (plz excuse the language). I've been constantly having problems focusing and paying attention. In fact, the other day I almost hit a car on my way home from work (I tried to merge lanes without realizing that there was a car there). Luckily no damage was done but still I was really freaked out, I'm scarred that if I continue like this something really bad is going to happen. Does anyone have any advice on how I can try to untangle my mind each day and stay more attentive. I've been thinking of sleeping for a few minutes in my car before I drive home from work, and maybe bringing a bag of candy or something sweet to keep me awake for away. Any other good tips that could help me out?