I wouldn't say it's selfish, but I do think it's important to balance the needs of yourself and your existing child with the desire for another.
As much as she would like a sibling I am sure that she also wants a healthy mum, and one that isn't suffering so much heartbreak too. I have a friend whose mum went through similar, and as a child it upset her that (to her childlike mind) she wasn't 'enough' - she does see it differently now, though!
I think if I was you I would be setting a fixed end date where I would no longer TTC. You don't want to spend her entire childhood on this IMO, and that's actually for you as much as her - this can't be an easy thing to go through.
I think because I'm struggling TTC number 1, in my mind I think if I had one child I wouldn't be willing to keep putting myself through this pain. It's the fear of never being a mother and the desire to be one that keeps me going. I think if I'm lucky enough for one then I will try for a second, but with a set end time. Wheras the end point for me TTC number 1 wil be either when I have a baby, or a doctor tells me there is literally zero chance. Until that point, I'm in.
For a very hypothetical number two I don't think I could bear to have such a black and white approach. I think I'd likely give myself a time limit and a certain amount of things to try, and once that was up I'd accept I was one and done.
But, it's a very personal choice. Only you know how much toll this is taking and whether you have the energy to continue.
I wish you the very best whatever you decide.