I know what you mean. I feel so good when i eat healthy and exercise. My excuses for healthy eating are usually, well I want to eat the junk... I don't really make excuses I just feel guilty about it afterward.
I honestly don't really skimp on the exercise. The only times I do are when I feel really time constrainted, like I have a big exam or a test coming up (as in a few hours away or the following day.). I like exercise and I find it usually helps with stress, so taking out 30mins for a run helps me feel better and focus more...
That said the hardest part is actually getting into my workout clothes. Somedays when i know I'll be tempted to skip a workout I get home and immeadiately put on my running gear... that way I'll be ready to go the moment I feel I should go. No excuses then.
EDITED TO ADD: Also I think we make excuses because we're trying to break a habit and because junk food, and television and that sort of thing provide a quick fix/relief/feelings of happiness/high.... And as humans we always go for the quick fix...
Your goal of 50 lbs in ten months sounds very reasonable to me. Good luck wiht it.
I'd like to be down 10-15lbs (15 at the VERY MOST. that'll put me at 132, which is a little low for my body frame I think.) by my next birthday, in october. Although I'd like to be down those 10lbs by the end of next term, april 2008.