I don't think I should do my coworkers work for her
Why wouldn't you do her work for her?
I prefer to work alone and make sure my own work is done.
You should make sure your work gets done on time before attempting to help others. It makes you look good to help others at work but you should not let them take advantage of you.
I already said I'm not going to force myself to do her work
I think it's great that you are standing up for yourself and refusing to take on someone else's work. People will learn they can't count on you to be taken advantage of.