I'm upset with my gf for constantly bringing up ex and their sex life
Why do you think she talks about her ex so much?
Because she likes to brag about her ex and because it makes her feel superior
I understand that hearing about her ex makes you feel hurt and uncomfortable. It's important to let her know what you think about her bringing up an old relationship.
Nah I like that she keeps bringing them up because it makes me feel like she's mine.
It seems that you are looking at this topic in a new way. That's a good thing if it does not cause conflict or resentment between you and your partner.