I see how you want to be respectful towards her and in many ways you are. Here's how this can go very wrong for you both though. I was once in your GFs position. I was stressed to the max with work deadlines that needed my fullest attention and my BF had quietly made up his mind that he did not want to be with me anymore and to start a new grad program overseas. He like you, did not want to stress me even further so he waited. I'll spare you the details but here are some things we both learned and we are still good friends.

 1) You know that you are living a lie until you break up so don't make it worse. Do not have sex with her, make up any excuse but do not have sex. You will be using her if you do and that would qualify you in the shit guy category, a place that your moral compass does not want you to go so badly that you are willing to put yourself out there on reddit and ask for help.  

2) Break up with her immediately after her exams. Don't put it off until the lease ends or picking a weekend to visit. You are wasting her time, and again by stringing her along "for her benefit" after the exams will once again be a form of using her. There really never is a perfect time for a breakup. Those conversations will always suck.  

Bottom line here is. Your intentions are noble but be mindful that you need to cut her loose ASAP after her exams because if you respect her, respect her time and don't let her waste it waiting and planning a life next to you for an instant longer than she should. best of luck OP.