As the others said here, GF + visiting her ex + ex still has feeling for her = trouble:trouble: :trouble:
Here's how this one is going to work. You're either going to tell her how you feel or you're going to walk away. She will not be visiting your ex as long as ou two are together (since it's not ok with you).
Maurizio brings up a good point in that you sound weak and insecure if you protest, but I think sounding confident and eloquent here will not show this. Say something like this to her:
"Look [gf's name]. I know I'm going to come off as a little overbearing here or w/e, but I am not ok with you seeing your ex, especially after he professed his love for you. Too many times I've heard stories of this situation and seen this sorta thing first hand (the latter may be stretching the truth),, and oftentimes it does not end well for the relationship. I trust you - make no mistake about that, but I do not trust your ex. Call me insecure and call me jealous, but the bottom line is I am not comfortable with you visiting an ex who still has feelings for you."
This takes the form of Eminem's rap at the end of 8 Mile. You basically already throw out anything she can call you (jealous, insecure, etc.) without even really saying you are those things! More importantly, if you say this with confidence and dignity, she will need to make a decision on the spot. See the ex, or tell him no? If she says she's still visiting the ex, you walk away. Yes it's a little drastic, but it states a point that her actions cannot be tolerated, and if she still really likes you then you will be getting a call very soon. If she says she cancelled seeing the ex, then you know she still likes you. However, if she jumps to go "hang out with him," you will see her true colors; action speaks louder than words.