It sounds as if you have the opinions of many others that what you attempt to practice is not poly... but thought I would add my two cents for what it is worth...
I agree that what you have practiced so far is an "open relationship." The fact that you called it Poly and then proceeded to have sex with someone even though your partner was struggling makes me think that in fact you were cheating on him... it sounds like you let him know your feelings about wanting to be free to love others, he told you his boundaries, you agreed to respect them, met someone, he felt jealous, nothing was resolved about his feelings and then you slept with someone anyways. This to me is cheating. If someone disrespects another by doing something beyond their comfort zone sexually with or without their knowing it is going to happen to me is openly cheating. To say you won't do it again is ridiculous and unbelievable. Perhaps you should be searching for other like minded people to have romantic relationships with or at least discuss that you are this way inclined before entering a relationship with them. Doing this would be respectful to them and to you. I am not convinced that you love S enough to make Poly or an open relationship work with him by the actions you have taken.
Poly to me is about respecting boundaries both another's and your own, this is how love is proven... it can never be proven by going with your horniness and thinking that just because you have said you are Poly means you can do whatever you want.
I would hope that next time you would take the time to get to know who you are wanting to be intimate with. I would hope you would take the time needed to make sure your partner feels respected, loved, cherished and comfortable with the idea. Lastly, I would hope you would take the time to make sure you partner gets to know your new potential partner and the idea of the two of you being intimate. Only then, when everyone is ready, should steps be taken to physical intimacy. This is what a Polyamorous relationship is to me. Otherwise you do come off as wanting a "fuck buddy" and are in fact a "play girl." This is fine, but call it what it is, not Poly please.
I am not meaning to sound harsh and I am not trying to be judgmental. I feel for you and respect that you are still young and that at your age it is important to try things out in order to find out your place within it all. I was so much like you when I was your age and can completely relate. Sorry too that I sound like an old lady (I'm turning 40 this year) that is inflicting my wisdom on you but I hope that you can take it as such, learn from it, make it your own and turn it into your own wisdom to pass on. You sound like a smart woman who knows what she wants. There is nothing wrong with that, but to do it in harming none and helping all is better all around for you and those around you... and I believe, the whole world... (but that is another thing