I usually don't give advice in chat rooms, but in your case....

I can identify with you because i went through the EXACT same thing.

I am the youngest of 3 boys. When my father passed in 1984 i was 19
and going to school. My mother , who was an educated professional, took it very hard. Even though we all knew that dad was in poor health, it was a shock for her.

Over time i noticed that her attitude towards me became more possessive, especially with my dating life. It got increasingly weird, to the point where she would actually sulk if i had plans. One time she even disconnected the phone and hid it because i was "infatuated" with a new girlfriend (1988, before cell phones).

Things calmed down for a time. then, in 1994, she became very ill and almost died. I moved in with her because she couldn't live alone.
Things only got worse. I actually gave up dating because it would make her so upset.

In 2002 she died from her illness. Since then i have had a few girlfriends
and even lived with one for several years. But , i believe the woman i was meant to be with, i met when i couldn't date anyone, because it would send mom into fits.

When your father passed, your mother simply changed your job description from "son" to "life partner"

You had no say in the matter, because in her eyes, you are a possession, not an equal. She has manipulated you into a situation that has brought you here. She has exploited your loyalty and used guilt and tantrums to get you to placate.

You give in to this because you love her and in your mind, she is still the ultimate authority figure.

If you truly want out, start by finding a place to stay. A friends couch for a weekend to start. when she flips out (and she will) remind her that you are a grown man and not her dog. Eventually she will either accept this, or she will change tactics. Be prepared.

Can you move in with a friend?

Finally, talk to your ex girlfriend. Explain the situation and how she was just an innocent bystander.

good luck!!