I am glad it was only in my head that this online poly community is a little prickish to couples
Of course you'll miss her, and it's OK to feel your emotions. You have them for reason and I hope you keep coming back here to write about them because if you don't have someone to talk to it can hard. Whatever you do, I definitely wouldn't ignore them and become detached because someone jumped to conclusions because you weren't careful enough when wrote them out.
It would be awesome if you really could have one foot in past, and one it the now, but it might only be satisfying if you had a dick so when you pissed you could arc it over and into polyamory dot com every time someone jumps all over you because of something some couple did in their past.
Give yourself break, I think you are doing pretty damn well. It takes a lot of courage, a lot of love, and a lot of understanding to back off like you have and there is nothing you can do that will instantly ease your sorrow, or soothe your sadness when you miss your ex-girlfriend. Give yourself credit, there really isn't a harder situation than to have your partner continue seeing them. But don't quit now, because if you make it through this you can make it through anything.
Not to get your hopes up or anything, but just know that your ex-girlfriend and your spouse will realize how much love and understanding it takes to fulfill your role in this. If they are wise enough to do the little things to help you work through this, and you all recognize each others effort, you will have a spouse and a friend that no other relationships can ever compare to. And after a year or so, if you can hold it together, you might truly understand that friendships that strong will make you one the richest people who ever lived. Friends like that are priceless, and it sounds like you are willing to be that friend to her. Hopefully the two of them will recognize that.
I am sure it's hard feeling alone, but it could always be worse, and there is nothing that makes it worse faster than getting involved with the wrong person in any type of non-monogamous relationship