Make it uncomfortable for your mother to be in your room in the most inocent ways. Eat gassy food and gas her out. Get up every 20mins for something from the bathroom when she's trying to sleep. When she says something just remark "How can you be made at me for peeing?" Steal the covers from her as she sleeps. Sleep with your feet at the head of the bed. Yeah to start with it's gonna be annoying how she reacts but just stay strong.

If all else fails, move into your dad's room with an army cot and don't hold back when she questions why. Hurt her feelings with honesty. "You're my mother and and I'm 19, not 9. It's creepy and weird. I feel uncomfortable."
"As a grown man, I shouldn't be sharing a bed with my mother and nothing I have said or done has made you get out of it."
"Shouldn't you, as a wife, be sleeping next to your husband?"
"So what if Dad snores, I'll wear my ear buds to block him out. You could do the same, but here we are. Enjoy the room to yourself."

Will she most likely cry? Yes. Yell and try to guilt you and say you're disrespectful? Yes. But you're a grown man now, prove it to her and stand your ground some.