I agree with AloneICannot in that there are numerous factors to be considered and no-one here can give you THE answer, only speculations.
This is very common and very possible:
"like you don't trust that she has the ability to do it on her own. Yes, girlfriends LOVE a caring, helping boyfriend. But when it's overdone they begin to feel belittled."
Other possibilities include the importance you are placing on her achievements and the passing of this exam. Of course such things are important, she knows this, and she may feel that her importance to you is based on how well she does in life, in school, in her achievements rather than her individuality and heart.
Exams and events that influence or guide our lives are extremely stressful. It is far easier to put too much weight on them (feeling like these things are the end of the world) rather than too little weight. You may be compounding her stress by adding your own, and her having to worry about your stress over her life in addition to the personal chaos she already feels. I am sure she does not want to feel like she is a point of stress in your life.
If you are asking her too often, perhaps she feels that you do not trust in her abilities of self-insight (like AICNS astutely mentioned). This overpowering shadow displays a mistrust in her word when she tells you there is nothing more for you to do. Healthy women do not want a father, they want a partner. You are one of her escapes from the drudgery of the world, but perhaps what was once her sanctuary is now turning into a boot camp?
My suggestions would be to ask what you can do to help and when/if she tells you "nothing" then trust in her word. You can still do things that will help her and show your support; such as not forcing a study schedule that doesn't work for her, bringing her food while she studies, asking her how SHE is doing (emotional and mental health aside from schooling), does she need extra pens, quiet time, letting her know you are there but not over her shoulder, etc. Your support is what will help her the most, not trying to do it for her.
Focus and remind her of how capable you know she is, on all of her strengths and the confidence you have in her, not on pointing out any academic weak points you perceive. If that is a dynamic that can be found in this situation, I fully understand that it can feel like focusing on the weaknesses will result in strengthening those areas but it most likely will have the opposite effect. When I am feeling weak or have an upcoming trial, it is my partner reminding me of how capable he perceives me to be that makes me feel I can take on the world- more importantly it is that which motivates me to be work on becoming better.
I can tell you really care for her, and I understand first hand wanting to be able to GIVE someone peace of mind/confidence/wonderful things when you care for them. I wish I could make everyone in the world as happy and carefree as children!!! But, sigh, we're only human.
A cornerstone of addiction is having issues with control. You can use her exam and this situation as a wonderful exercise in relinquishing the control so many of us try to grasp at. Control of the situation- ie. desperately wanting her to do well, and doing everything you can to make it happen, to try to control the outcome. Take a breath, step back, and find a state of nirvana where you trust in each other, trust in your God (if you are a religious man) to do what is best for your life path, and in yourselves in that no matter what happens everything will be OK. Do what you know is the best thing to do, to the best of your abilities, and not to worry about the final outcome. It truly is the journey that matters!
Good luck to both you and your girl! I hope she does well