I’ve been through this same situation.
You say: “my depressed girlfriend left me. Her reasons were she didn't want affection, she wanted to set me free as she felt like a burden, I deserved better, she felt guilty for holding me back and for giving no intimacy (literally none, not even a kiss or cuddle for over 2 months). I have since discovered a lot of things that make me question her reasoning. I discovered she had been actively looking for sexual encounters with men, women and couples in all kinds of BDSM and Fetish websites and domains, in the month before our break up. “
1. My now deceased wife was sexually abused. (Has your former girlfriend ever said anything about that in her life?)
2. What can happen from that, is that they become a Borderline Personality Syndrome person, in which their ego divides from the trauma, with the trauma being on one side of the division, and what’s left of them, at that age, 5, 10, on the other side of the trauma.
(These are all my opinions. I would encourage you to look up that condition on the net. I am bipolar, myself.)
3. They become like two people, but since they are only one person physically, everyone thinks they are one person. They are only one person, but mentally they can flip from one side of their divided ego to the other.
On one side, they are angels, you know, the 5 or 10 year old mentioned above. On the other side, they are the complete opposite. Since they learned sex early and coldly, with no emotions, that’s what this other side tends toward: cold sex.
4. My wife did like your girlfriend. Was all devoted to me, since one half of her needed stability, like any 5 or 10 year old, and to have someone to come home to after a one hour fling with a complete stranger. So the one hour fling is the other side of her divided ego.
5. You said, “She since has become a submissive to a "Dom" man and has a submissive man of her own in this bizarre fetish world. I discovered she had started to resent me for things such as taking her for granted, which we both admitted we had done and were working at”
Like my wife, once she finds another lover or lovers, she may not need us, and ditch us. Or, somewhat like my wife, when she gets caught running around, and has no choice (since her stable person has found out about her), she slips over to the other side of herself and attaches herself to this other lover, or series of lovers.
Should they dump her, she may come running back to you, but since they may have so many lovers, that might not happen.
6. You say, “The most confusing thing for me is the lack of understanding I have of my own mind. Before all this I was very level headed, could control my thoughts and act on things logically, I could understand them. Now I don't trust my own instincts.“
The reason you can’t figure her out is because she’s two people, and nobody can figure out two people in one body, especially when they don’t know what’s going on, that in her mind she is two people.
She presented herself as one person, and when that one person disappeared, you doubted your mind. Your mind perceived correctly. It’s what you were looking at that was incorrect or distorted.
7. You said, “On one side I still love her, care for her and miss her. I want to help her,”
After my first wife passed away from a long illness, I met a woman that I was also trying to help like you were with your girlfriend. For six months I’ll say she was miss help me. And I did. One day she wanted to borrow some money, and she never paid it back. Never intended to. Never was really sincere of needing and wanting my help.
Well, one half of her was sincere, right. But when she needed something, she flipped to her other side, and became a con artist. Right, when will I ever learn? Exactly.
8. You say, “I can see that this is what she does when she becomes depressed (mirrors a previous depression episode years ago, where she became promiscuous and then felt disgusted with herself once the episode had passed) and want to support her so she doesn't feel she has to sink to these depths”
She’s flipping back and forth. From good little girl, to horrible bad girl, cause that’s What she is.
9. You say, A. “On one side I still love her, care for her and miss her. I want to help her,”
B. “But equally I feel like I am beginning to hate her, I am disgusted by her. That she would treat anyone, let alone me, the man she told was her soul mate, like this.”
There again, are her two opposite but equal sides.
10. You say, “I constantly wrestle with thoughts and images of her at these sex parties I know she signed up to, in intense sexual situations with multiple partners while we were together and I cannot control them”
I had the same feelings with my wife. She would occasionally give me details of her other life. Yeah, like on her teacher’s desk at the end of the school day with her lover, a fellow school teacher. Yeah, like in a threesome in a motel room, with the other partner sometimes being a male, sometimes being a female.
Yeah, right, that can drive you crazy, which is exactly what she wants to happen, when she’s not being your loyal girlfriend that is.
You’re actually in pretty good shape, because you’re not married and you don’t have a child.
Try 29 years of finding out about her outside sex, but in a marriage, buying a house, with a child. After you’ve been through 29 years of that, then you and I can talk about girlfriend/wife trouble. Until you’ve been through that, until you’ve been to the heart doctor with chest pain numerous times from all of that, you don’t really know trouble.
So actually when she’s behaving badly, yeah, when you hate her, right, that’s when you need to get away. Act on that. Leave.
I use to complain to my psychiatrist about my wife’s problems, after my wife died of a long illness, and she would be telling me to forget about it. Only when I met another, right, borderline, did I forget about the first borderline.
I now have a home health care worker who helps me a couple times a week who helps me with conversation from time to time, which helps me forget about the other.
These are all my opinions and experiences.