you can always be honest and pour your efforts into your work and into taking care of yourself and just tell her "I don't have the time to spare right now, but hey, maybe, I can text/call/etc you when I've got some spare time, work is just pushing me soo hard"
bit of a warning.. this is a method I use to friend zone someone that I'm not romantically interested in and has worked very well in the past, but that sounds like where you want this to go for now.
best of luck