It's a strange one because you presumably know your gf better than the descriptive words we assess in these written posts?
I make no differentiation between being 'open' in a relationship or expecting your gf to be monogamous - If you undertake behaviour in a relationship that offends your partner or makes them feel like cr%p then honestly, you just wouldn't do it.
However your behaviour is to the point of controlling your gf then there is a problem (this is the other way round)
I don't think you are overthinking this. At this stage you should be considering your gf 100% and vice versa. The age difference isn't so much an issue as whether it has given her a sense of power and she is acting out to gain adoration from others (including you) I know you probably won't like my answer but I think she thinks more of herself than of you. I'm sure if you were to discuss leaving she would put on a great show of distress, but deep down she is playing you, wants you to change your mind and stay with her. If it was a man you would say he is 'playing' the field.
Please do put your own needs first, and have the confidence to leave this woman to her own devices. Imagine if you had that relationship with someone who gives you time and space, but whom you can trust absolutely. You can both agree what makes your relationship and this is key you will feel on cloud 9.
Here's to cloud 9's.