I've been there too. It's difficult, but you're not a bad person for feeling this way. Just because your mother has a serious problem, that doesn't make your own problems any less difficult to bear. You're only human.
It might help you to know that the operation you're hoping to get is not a mastectomy - what you're looking for is chest reconstruction, which has some similarities to a mastectomy but is definitely not the same thing. Your mother is having an entirely different operation to the one you want, and the results will not look the same at all.
I hate to tell you this, but a mastectomy can be destructive and disfiguring but she needs to have it in an attempt to save her life. The operation you want is a beautiful, self-affirming operation that will improve your quality of life as well as your appearance. Two different ops, two very different outcomes.
My mother told me through tears that her mastectomies made her feel invalidated as a woman: she felt her femininity had been damaged & undermined by the loss of that particular feature of her body. I found that surprising and it was a completely alien concept to me, but I could see how much it hurt her so I just tried to console her. I never told her how I felt about it because that would've been a slap in the face for her.
Sorry your mum's unwell. I hope she recovers soon.