You can't get her off your mind.
The best way is to tell her that you have treatment coming up that you need to be focusing on before spending more time talking with her.
You will see how she reacts. Hopefully she'll support you in that decision. If not, then it'll be much easier to get her off your mind, because then she sucks, lol
Then you simply have to accept that she's on your mind. Notice you're thinking of her, and gently shift your thoughts away to other things. It'll happen often, and it'll be distracting, but that's fine.