I'm going to give more effort and attention to my girlfriend and job than my mom
Why would you do that to your mom?
Mom does not need my attention or guidance
I see you're point, but it's important to balance family life and romantic relationships. Your mom might be upset if you don't give her any attention?
Why do I have to support my mom at all? I have supported her already in the past.  I'm thinking about how to break the news to her.
Just because you are older now doesn't mean you can't still have a relationship with your mom. She supported you as well, right?