My mother makes me feel like I'm the child and she's the boss. It's my house and yet I still have to answer to her like I'm 15. I hate it.

" I'm an adult mom. I will conduct myself as an adult and no longer answer to you for my whereabouts or actions. This is my home and you will respect that." etc.
Or could be she is bored and living vicariously through you.
In any case you need to set some ground rules.
You are an adult and deserve to be respected.
The roles have not necessarily reversed but she needs to know she is not the boss any longer.

Yes, she wants to know where I'm going and who I'm with, what time I'll be home, who was that on the phone? I can't take much more of this!

Is this a matter of "Where are you going?" and "When will you be home?" or is it more like "You aren't wearing THAT, are you?!" and "Finish your vegetables." I have to remind my mom (and she doesn't live with me) that I am 49 and can make my own decisions. I would answer with "Why yes, I am wearing this. I bought it, paid for it, and will wear it." or "Mom, I had a big salad for lunch, therefore I chose not to finish my vegetables with dinner. Please don't worry about me."

I would think it must be hard for an elderly parent to give up being a parent even when living in an adult child's home, even if they need care and assistance from that child. I believe it goes against our intuition be "parented" by an adult child as well as being an adult child and having to "parent a parent'! I am pretty sure my parents will be pushing my buttons until the day I die! How does one stop being the mama?!

....then do not treat her as a guest...she is your tenant or roomate and needs to follow the "house rules." I did this a lot when my mother first arrived to live near us. I was hopping all over the place doing things for her - her way. Now I figure out what her needs are - the wants come when I have time.
Also, I found that I was not communicating with her appropriately. For example, I would fume over something thinking that she would get the "hint." Finally, I just spoke my mind as calmly as I could and she finally got the picture. The fact that your Mom shares quarters with you makes it even more imperative that you have open communication or you will just continue to be resentful and lose yourself in the midst.
good luck,
Lilli