welcome alienagent! I can totally relate to not wanting to be the cause of additional stress in a parent's life. Don't want to give them a coronary, either! LOL. which is why I have let them push me around all this time, too. I'm not in the exact situation as you are, but I would think, if your mom is ok with living with her disfellowshipped daughter, which probably was a lot for her to handle when it happened, she might be able to deal with you leaving the religion, too. Maybe it wouldn't be so hard for her if she thinks what took you away was the girlfriend, instead of having doubts. Doubts scare and threaten them. They are always fighting doubts and don't like them rubbed in their faces, although they quite deserve it.
This is the life your mom chose, so she should have to deal with the consequences of her choice. She knew when she made that choice, her children might not follow, so she made the decision to make that choice anyway. Let her be who she is, and don't worry about her, she made her own bed. If she can't let you be who you are, that's her problem.
If I was in your situation, I would offer to help her financially, if you can, after you leave her home, regardless of your standing in the congregation. If she doesn't want your help, so be it.
And here I am with all this advice I can't bring myself to follow! LOL.
Good luck!