No, you're not overreacting. I don't mind hearing the occasional story about an ex, but if it was this frequent - and against my wishes - I'd be getting quite frustrated. How long ago did she break up from him? I'm wondering if she's not yet over him.
Anyway, the long and short if it is that you've expressed your wishes and she's not respecting them. Now, I rarely suggest 'breaks' but I think this would be an ideal time for one. Tell her that you need space from hearing about the ex all the time and that she's to stay out of contact with you for a bit. You can also say that you will only get back together when she starts to respect your needs.
Having said that, you have to follow through on any ultimatums like this. But if her talking about him all the time and ignoring your wishes doesn't bother you enough to walk away, then you need to learn to accept it.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.