Hi Chupacabra (good name)
As GG said - there's no right or wrong. You feel the way you do.
I can certainly understand how you feel. You're not being unreasonable.
It's not really a good sign that she's made an agreement and then tried to swerve it to suit her. We are all human and we all make mistakes. It means that either she's a bit of a pushover and will make agreements in the moment to please someone, even if she knows that they don't suit her. Or, on the worst side, she wants what she wants and she's not entirely moral or honest.
That being said... she could have gone ahead and started dating the ex again, without telling you. So not all is lost.
I think there are a few things to consider here.
For me, if a primary/secondary model is adopted, it's going to take time.
To me... having a primary means that I value and cherish a particular person SO much that I actively *want* to make them a priority. Not because they have the title of being my girlfriend - but because they are compatible enough with me that I want to spend my life with them. I want to commit to them - so I'm willing to call them a primary, a priority, and consider how everything I do might affect that important relationship.
I wouldn't make a new lover a primary, if I were single. In my opinion, it's too soon.
For me, the idea of primary (priority) is completely defined by the idea that you have had time to want to make that commitment. It's a lovely idea to call someone a primary when you first start dating - but how is that going to work in practice? How are you going to want to put their needs above yours, when you haven't yet had enough time to even know if it's going to be a long term thing?
The issue of her ex complicates things.
I do feel that we can't force roles in people's lives. But roles can change by themselves. When I first met my girlfriend, she had a husband and a very important online partner. I never, ever thought I would make it to a primary level... but I fell for her so hard that I just wanted to be in her life. Over time, I became her primary. I became very important after just a few weeks - but to become a real, genuine, completely committed primary? That took at least 6 months - probably more like a year, in reality. Does that make sense?
From my perspective... whatever she wants is ok. Whatever you want is ok, too.
If she wants her ex (or rather, wants to see if they can work it out), that's what she wants. In my opinion, it's not really fair to say "nope, you can't have what you really want, because it makes me uncomfortable". For some people, poly isn't about just being able to date multiple people - it can very well be about having feelings for more than one *specific* person and wanting to explore both relationships at the same time.
If I were in your shoes? I'd either let the relationship go... or put it on hiatus, whilst she explore things with her ex... or continue, but not under a false security of the term 'primaries'. I genuinely don't think that will work, for what my opinion is worth. My leaning here would be to give it space for a few months, at least, and let her work out her thing with her ex, so that you don't get hurt in the crossfire.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha