I don’t think you’re being unreasonable at all for wanting a night or two alone. I think maybe she interpreted your request wrong based on the timing. After you just spent two days without her, she may have felt like you were saying you enjoyed your time without her so much and now you want to have more time alone. Of course, this is not what you meant, but insecurity can really twist things in your head.

It sounds like you need a day or two to be introverted which is perfectly healthy. You need time to recharge and like you said, do things alone. Being with your partner 100% of the time is not feasible as you and can lead to lots of co-dependency issues. I think you should try and explain again but emphasize that it really has nothing to do with her at all. You love her and want to spend time with her, and you will miss her when she’s not around. You simply just need time to be an introvert and recharge for the sake of your mental health. Listen and try and empathetically explain.

If she still can’t understand your reasoning, I think it’s in your best interest to just be firm and say that’s what your doing, let her decide if that’s a dealbreaker or not. Your girlfriend should be able to respect your mental health and also your studies. Relationships always feel important but don’t sacrifice your well being and let it get in the way of your trajectory.