Everything that has already been said, I thoroughly agree with. Your exams must take priority at this time, and expressing the importance of this with your Mistress as soon as possible is very important.

I suggest, however, discussing a schedule for the time that you commit strictly for Her as well as a set time for your studies (where you are not to be interrupted). This way, She will recognize that you are maintaining your committment to your relationship, and not feel "put off".