Hi guiltier,

You're not a lousy anything
No-one can be in two places at once, or be everything to everybody
Right now you have commitments you need to take care of, and your dad has support. They can call you from the hospital if they need to, and I'm sure if it turned out you were needed there you'd talk to someone at the school and tell them you couldn't stay
And just remember, all that's really important is that your dad is where he needs to be and should be in good hands.

As for the rest..........I 1001% agree that you need to put yourself first at times, and if you need to say "No" then it should be respected by anyone who cares about you........the one's who show that they don't care about you.........well why worry about them so much??!!
But time for the one's who care to do a bit more for you, I'd say, if maybe you could let them know a bit (more??) about how you're feeling

Alison