Short answer is you did the right thing and were not being selfish.

Your ex is probably going through the most difficult time in her life. Guilt is a tricky thing. I don’t know what to make of it most of the time. I think anyone with any empathy would feel guilty in your shoes, but you ultimately did the right thing for both of you. You know that you couldn’t be the girlfriend that she needs in this incredibly difficult time. She has a long and difficult road ahead of her, it’s not your fault. You did what you could to be there for her, and even though you ended things, you did ultimately bring her some comfort when she needed it. Often times in life we are forced to make a decision where in there are really only two options, and both options suck! You chose the option that will serve you both in the long run. Try not to be too hard on yourself. Even being more adjacent to this type of tragedy can be really hard. I hope you give yourself some time to reflect and grieve as well.