Thank you for that. But it impossible for us to find the full story and there are abstractions in there - as in evasiveness. Specifically:
"That time I was told a few times that I was wrong to blatently and openly enter into every relationship like it is just a game. So, I decided to experiment with my then GF to build a relationship on a cooperative basis, where we both define each-other's identities. She was very enthusiastic about this, and we went ahead with it. (And she did define my identity, although I don't think I defined any of hers.)"
There are many topics my wife literally can not think or talk about. A unique nature she's had her whole life. So I have to do A LOT of reading between the lines - and I do get it wrong some times - badly - but reading between the lines on abstractions is what I have to do.
THIS: experiment with my then GF to build a relationship on a cooperative basis, where we both define each-other's identities.
It sounds like partner swapping, swinging, 3-some or something else sexual. If so, that really factors into it and might have something to do with her getting all pissed off at you and stabbing you in combination with you trying get her off of pills.
Your logic doesn't work. You didn't get stabbed again because you left her - or she left you - not because you screwed someone else. I mean, really.
But thank you for sharing, really. We aren't perfect people. Ideally we not only learn by out experiences, but also others. However, of our experiences we should examine them not just from our own inner eye, but as a remote 3rd eye looking in. Since you opt not to explain your abstractions, it sounds like you took some relationship gamble for whatever reasons and it blew up on you.
Which leads to another comment/advise. MOST people usually don't always mean what they say, particularly if YOU make the suggestion and the other person says "OK." That doesn't mean the person really agrees and in the end it doesn't work out.
A common example is a man proposing a 3-some OR spouse swapping, and the wife says "ok" - but its not REALLY ok. And then its going to go badly from there. Not saying that's what happened, but your statement of "I decided to experiment with my then GF" sets off alarims, because of the "I decided" - rather than "we decided."
I see no reason to be judgmental about anything you wrote, BTW. You weren't married nor does it sound marriage-like. Depending what it was and assuming "consent," "experimenting" is usually ok. I dunno. Maybe it was surprise anal sex or some other sexual thing or even gender orientation thing - and she didn't care for it. I don't think it matters. Once you were retaliatory screwing someone else and she was stabbing you it was pretty much over.