What can I do if I have a 3 year old little girl with someone that I just truly do not want to be with anymore?

I am a 70% disabled vet getting paid at 100% for the rest of my life so I don't have to worry about the work thing. She only works 1 day a week and has a GED. I am afraid if I leave this girl that I will only get partial custody I want to push for as much of the custody as possible not for myself or to hurt her but for my child's sake I feel I can provide her more in her life. I don't want to keep the mother from her. I just don't want her to have to live around such an unstable environment. The mother has severe depression. She has a bad attitude and will try arguing right in front of the children. I do not want that kind of environment for my little girl. Anyways small details are I have stable income and a brand new car in my name. The mother has no vehicle and only works 1 day a week. She depends on her family to pay all her bills.