Well, considering that you are still young, there really isn't a whole lot you can do. Just gotta hang in there. As the saying goes "Time spent apart, makes the heart grow fonder." :)
Well, you just react with the issue, if you tell your boyfriend/girlfriend then they won't mind. Otherwise they shouldn't deserve to be with you.
Why is it so hard? He is your boyfriend.
Call in Batman.
Your parents are far more important than your boyfriend. If he can't understand that you have to take care of your mother an support her then he isn't worth worrying about.
Then just tell him how you feel. Communication is key to a relationship.
You'll probably feel better afterwards though. No more keeping it secret. Hopefully he respects your feelings.