Can I just say...I admire you for wanting to go into engineering? I've taken biology, anatomy, chemistry and gotten all 'A's but I failed physics last year. so uh...yeeeeeah. :p But it does sound like you are frustrated and REALLY stressed. Which are obviously making things ten times worse.
You make it clear that you understand the work...yet you still have trouble with it (or something. I might have interpreted it wrong.) so maybe if you get someone to work with. a teacher or another classmate it might get easier/ more productive?
And you should sleep more if possible. Because sleeping relieves tension and stress (not to mention anxiety).
and it really is worth the effort, it really is. :)