It is extremely difficult (or even impossible?) to set boundaries with controlling parents when you are financially dependent on them and living in their home. Your mother does not have a right to determine who you date. Nor does she have a right to block you from leaving the house. But she believes she does. She may perceive you as an extension of herself, rather than a separate entity in your own right, therefore to her your choices seem like her choices. I doubt you can convince her otherwise so you need to physically and financially separate yourself as soon as possible.

I recommend developing a plan to leave her house and live elsewhere....whether that means working and saving money for a while, searching for a roommate or two, or seeking some sort of professional support and/or government assistance. Your mother's perception is that she owns you because you live under her roof and (I assume) she pays the bills. I think the only way to end her control is to live elsewhere and support yourself as best you can. Though of course I don't know any particulars of your finances etc

20 is a good age to be living without one's parents and supporting one's self....choosing your own partner and lifestyle etc. Do you think you can find ways to do that?