An update to my story:
We were at the mall buying stuff for Christmas. She saw this cool watch along the way. Out of no where, she brought up again her bf. I was pissed off and handled myself properly. In a calm tone, I told her,"That's nice. Why won't you buy that for him. After all, it's Christmas right?" She was a bit confused and we walked away from the watch. When we were at the cashier's area, I saw my EX GF. I said hi and greeted her a Merry Christmas. I introduced my GF to my EX. My EX and I were talking like friends as we were falling in line to pay our stuff at the cashier. I noticed that my GF is silent and just smiling. My EX went off because she is meeting somebody at a cafe. We said goodbyes.
It really felt good that this time, I had a bit of revenge to my gf. I hope she won't stop bringing up her EX again..