When it comes to sex you should never, ever do anything you don't feel comfortable doing. It doesn't matter if you're in love with this person, doesn't matter if they love you, doesn't matter if you're married, doesn't matter if you owe them something, doesn't matter if you've already had sex or said you would, doesn't matter if they give you an ultimatum, doesn't matter if it makes them feel unwanted.
Nobody has the right to tell you what sexual activity, if any, that you should be doing. If you do not feel comfortable with sex yet then do not give in to what she asks. Be honest and tell her you just don't feel ready yet, if she's a nice, understanding person who cares about you she will respect that. If she isn't she will try to force you or guilt you, and to be honest, it sounds like she already is trying to make you do it when she knows you don't want to. I would personally break up with her if she actively tries to force it out of you. It's fair enough if it does make her feel unwanted, that's what happens for some people if their partner doesn't want to have sex for whatever reason and it isn't unjustified, it just doesn't mean she can try to make you do it for her benefit.
All you can do is be honest and explain your feelings. I would say her reaction will tell you important things about her character.