Yes, you are allowing others to take advantage of you and you are the only one that can put your foot down and stop this. If they haven't paid you, send them a letter and advise them that you are letting their children go at the end of the week or the end of the month, but don't commit to teaching the come March 1st w/o a check in hand for both February and March.
I know you feel sorry for the little girl, but you aren't running a taxi service and gas is expensive, more so when you don't know when you'll get paid.
I think it would do you a world of good to get away for a few days and go to NC w/your mom. The change in scenery would really help your pma.
As for the difficult boy, did you look into some of the suggestions I gave you?
Heather, you have to learn to say "no" and "I will not allow others to take advantage of me". There's nothing wrong in saying no, especially when you know that you are being used over and over again. Time to change all of that. I know you care for your students, but that's not paying your bills.
Take care.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.