My girlfriend has been diagnosed with life-altering medical condition. I want to break up. How can I?

My girlfriend, of just over a year, has been dealing with chronic stomach pain, which doctors have said is untreatable. The past four months have been hell for her, and draining on our relationship. Everyday now I think of how daunting it is that I will spend the rest of my life with this woman, and she will not be the girl I fell in love with. The condition has left her depressed, and with little strength or motivation, and has left me frustrated. I feel it is best for myself to get out of this relationship, but I worry of how this will impact her. I need advice.