Experience is meaningless. People learn the wrong lessons from experience all the time. Saying "I have experience" as the sole support for an argument is even more meaningless.
In order for it to mean anything, one has to demonstrate that their experience was meaningful. I see this all the time with non-monogamy. Someone comes along and says "I had a threesome once and things turned out horribly" and infer from this that all people who choose non-monogamy are doomed to repeated failure, completely dismissing that lots of people are successful with various forms of non-monogamy, and that strictly monogamous relationships fail all the time as well. I listen to people that establish credibility.
No way do I have time to respond to everything that's gone on in this thread, but I did want to comment on this.
Although I see the point you are trying to make, the threesome example is terrible. It's like a straight man saying, "I had sex with a guy once, it was awful." Number one, it's absurdly reductive (turning a complex identity into a single sexual event), and secondly, it's someone commenting on an identity they don't share. It'd be like me pontificating on what it's like to be a lesbian, as if I have any Earthly idea beyond the accounts of friends and strangers. What you're talking about has nothing to do with experience. You seem to be assuming no one here has experience because they haven't spelled it out for you, otherwise you are invalidating their experiences because they don't paint the picture you want.
Reading through this thread, you talked about being poly a few times, but being poly seems to have no bearing on what's going on here. As far as I can tell, you aren't involved with anyone right now, and your issues with the girl in question don't revolve around non-monogamy but the role of her ex in her life. I'm sure others here have been in that situation. Even I have, myself.
You seem to be aware that your jealous overreactions are unwarranted, especially since you aren't even dating this woman. From what I read in your last post, it sounds like you want to get to the bottom of his behavior. Ultimately, that is not your problem nor your responsibility. If you want to make a go of it with the woman you like, do it. If her ex interferes, you have a few options:
1. Ask her to talk to him and tell him to back off and leave you alone.
2. Deal with the fact that this person is part of her life and you have to figure out how to navigate that.
3. Don't be with her anymore.
Which one is not an option? "Sit down with the guy and have a little chat." If you are at the point where he is actively interfering in your relationship, the last thing that's going to help is you having a talk about it with him. Either he's trying to break you up, in which case talking to him plays right into his hands--it gives him an opening to attack you directly--or you are totally imagining what's going on, in which case he's going to think you're insane and probably tell her
you're insane and it's not going to work out well for anyone. Maybe it would be an option at some point in the future, if things are going well with her and he is still having issues with his new role in her life. Both
of you could talk to him and find a way to be friends. But right now? No.
Although there are situations where exes and new love interests can get along with each other, rarely does it work when it begins so adversarially. It requires everyone in the situation to be mature adults. So far, I'm not sure about the woman (don't know enough about her), but you and the ex (from what you've said of him) both fail that metric.
You must also be aware that starting a thread to discuss these issues while using such dramatic, over-the-top language would give people a very strange (read: negative) impression of you, right? You said you'd like to think that effective communication might work, but what I've seen from you in this thread has not been very "effective." It's been hyperbolic, defensive, hostile, passive-aggressive, and pretty much anything but "effective." If your goal was to rile people up and come off as some kind of jealous nut, mission accomplished. If your goal was to have a reasonable discussion of jealousy, communication, and building relationships, that ship blew up on the launch pad.
I don't think you're a bad guy. I think you're a bit too focused on your image rather than who you are, as a person. Instead of communicating honestly, you put on a show. I don't think that's beneficial to anyone, least of all yourself. You say you recognize your issues with jealous and possessiveness. Awareness is step one. What are you doing to change it? How are you planning to proceed with the woman you like, if you intend to proceed at all?
If you want to discuss any of this, discuss it. Two pages of you going on the counterattack against people impugning your fashion sense and personality traits has been really tedious to read.