I haven't been here in months, and probably don't have a lot to offer... but would saying something like this help?
"Mom, I love you and want to help you. You know about my work schedule now, and if I don't adhere to it, X happens. What would you suggest I do if you were in my shoes?"
This is easy for me to say b/c I am very close with my Mom and know she would appreciate all the factors.
That said, I have certainly cut out certain ppl in my life because they cause me more pain than they give back in friendship. Not that it is a tit for tat kind of thing... but I hope you know what I mean. Some people drain you and some add to your life and most do both but the equation balances.
One person I know told a certain long time friend of hers:
"I have worked a long time to get to a healthy, functional, happy place in my life and with deep regret I must tell you that I cannot afford to be your friend anymore. This relationship is threatening the healthy place in which I currently reside".
I don't know that I could ever say that to a relative, or even a friend, but I hope if the time comes it is appopriate, I have the nerve to do it.