I don't feel it's selfish. It's okay for someone to want autonomy and choice over the direction their lives take, especially if they are suffering.
If we were to stick with the concept of suicide as a "selfish" act. Is "selfishness" really that wrong? Is it really so awful? Surely everyone is selfish to some degree. Very few people do literally everything exclusively for others without ever thinking of their own desires, needs or interests. I actually think that it's okay to not be selfless, especially when it comes to your future and the rest of your life. Because ultimately, you are the one having to live this life. Not your family, not your friends - you.
If I'm selfish for wanting to have a choice about my own life and wanting to die with dignity on my terms instead of dying from illness/old age/accident/murder because that suits everyone else (except for me, the person actually living this life), then so be it.