I think you've gotten a lot of good advice and some other sides to the story.
Here's what I'm seeing.
Previously, you got to have what you want, and it seemed lovely. And the "thought" of her having another SO was fine, because it was theoretical.
Now it's a reality and you're experiencing some emotions you didn't plan on, jealousy, envy, loss, feeling replaced, feeling not as important.
Basically--all of the emotions most of us feel and have to work through when we're first learning to have multiple relationships (with a rare few exceptions of course!).
And instead of realizing "oh, this is what it feels like, wow! Why am I feeling this?", instead of investigating what emotions you're feeling and what thoughts (and make believe stories) your emotions are building... you just want it to stop, and want her to stop so you don't feel bad.
I totally understand that, believe me. I think many of us have been in that same place. But the fact is that your reaction/emotions/thoughts are pretty normal, and it is for YOU to work through them. Of course, getting that reassurance from your SO can make a huge difference, getting her support in realizing it's a bit harder for you than you'd like, so she can be reassuring and gentle and kind while you're working through it.
And maybe you're not getting the reassurance from your SO that you wanted because of your approach. Maybe instead of "I feel like shit, you need to stop what you're doing because it makes me feel bad", you could say something more like "Hey, I didn't realize that I'd have these strong feelings about you being with him after me. Can you reassure me that you miss me when I'm gone, and that we are still connected?"
You might get a better response, and more of what you need, if you ask for what will reassure you instead of trying to control her actions. Because frankly I do agree it's not fair to want her to sit there alone while you go home to your primary. And even if she were to agree to that, there would be a degree of resentment that would build that could possibly destroy what you have.
Besides, doing the work you need to do to work through your emotions/feelings/thoughts is important work. Yes it's easier not to do it and just try to make everybody else change their lives so that you don't have to. But in the long run, you're the one that loses out. Growth is good.