Sounds like you've got burn out to some extent. Or rather, you don't have allot of motivation right now.... which doesn't surprise me considering your other post and the fact that you're at a crossroads on allot of things, including your major...
When you have a career, you won't be like this, trust me... I.e. you will think to yourself "If i don't get off my ass and get my work done, i won't get paid, won't be able to pay my bills or eat etc... That is all the incentive you'll need
On a diff. note: Why on Earth can't you masturbate?? That is a normal and healthy thing, not to mention a stress reliever for men and women... so i'm just curious why she would ask you not to and why you feel like you 'hurt' her by doing something so normal? Creating such negative or limiting attitudes around sex/masturbation isn't the best thing for a relationship... because... you've gone and acted like a normal human being and feel somehow guilty...
Good luck
Sweetpea