Tell her when she’s not in a bad mood and you guys are in a comfortable setting (with no interruptions). Just say the truth; there isn’t really any way around it. If she really loves you, she will understand. She’s your mother after all!
As for moving out, look for a roommate(s) so you guys can pay rent together. You can work and save up in the meantime until you can live on your own.
I haven’t been in this situation before so my advice may not be as helpful but I hope things will turn out well!
after they bury Michael Brown today
let us remember that the fight for justice is not over
please do not forget Ferguson
please keep Michael Brown on your mind, his family and friends in your hearts, and the city of Ferguson in your prayers
let us continue to stand in solidarity with our brothers and sisters fighting for justice & peace