I arrived at polyamory via having a close friendship with an ex whom I considered my best friend. (Or rather, I arrived at polyamory after that relationship imploded and I realized I could have understood the situation better and communicated better with him using the framework / labels / structure of polyamory).
So I get what you mean about feeling that your girlfriend has a somewhat polyamorous relationship with her ex, even if they are not necessarily sexual anymore.
It might help you work through your jealousy by approaching the situation with poly in mind. The best friend/ex is in her life to stay, is really important to her, it's a relationship you should give her the freedom to keep, etc.
The basic idea of polyamory is that someone can have feelings and closeness with more than one person without diminishing the value of those feelings and closeness. Your girlfriend can love both you and her ex (maybe in different ways) and that does not diminish her love for you.
And also, you can COMMUNICATE with your girlfriend about her ex, as poly people do about their other relationships...you can point out how frequently she mentions the ex, for example, and say that you support the friendship completely but it also makes you jealous to hear about her CONSTANTLY.
However, I'm confused by part of your post. Your girlfriend may have a poly attitude toward relationships, but what does that have to do with whether YOU are capable of dating more than one person??? You can accept her relationship with her ex, and learn all about poly if it helps you understand her better, but you can still be monogamous yourself. You don't have to run out and date a bunch of people just because your girlfriend has a special bond with her ex.
Single, straight, female, solo, non-monogamous.