I don't want to stay with my gf following a serious chronic health diagnosis
What is the basis for your thinking here? Why would you not be willing to stay with her?
My gf got breast cancer and I don't want to stay to support her
You shouldn't leave someone just because something medical came up. It's wrong to want to leave someone when they have just been dealt a bad hand. They need you the most in the moment more than ever.
I'm in for a stress free life with a new fresh girlfriend.
You should never leave a relationship at a time when your partner needs you. One day you will find yourself all alone if you treat others this way.