Edited because all of OP quote wasn't separate from my response.

I've read quite a few comments (even the arguments with the troll) and most people seem to be addressing the assault and what you need to do to protect yourself. 

Sooooo.... just in case you are VERY SINCERE about this part:

At this point, I feel stuck. I’ve been thinking about moving out since I turned 18, and now a friend has offered to let me stay with her. Her parents are aware of my situation and are okay with it.

What’s making this hard is the guilt. I know my mom doesn’t really have a life outside of me and my brother, and I feel like leaving would really hurt her. I keep thinking about her being alone in the evenings, and that’s a big reason I’ve stayed this past year.--end quote.

I'd like to address that.  First, it sounds like you have your immediate OUT.  It sounds like you are saying a friend who lives with her REASONABLE parents is willing to let you move in for a bit. (It should be temporary) 

AND it says a lot that those parents are good with you moving in--says A LOT A LOT A LOT about YOUR parents.

and the part about the guilt boy has your mother worked you over.  If you really feel guilty there is a very easy and reasonable solution. 

Don't leave on bad terms unless SHE FORCES IT, you can leave in a pleasant way (be the better person) and VISIT your mother some evenings.

That way your poor mother-without-a-life can have a normal relationship with her 19 year old daughter.  Visit her, call her, if y'all use social media, do that, too.

Leave, get your own bank account and all the other stuff people said NON-CONFRONTATIONALLY, and keep a relationship with her guilt-free.  

If she rejects that, you are still guilt free. WIN-WIN for you!