Im dating the most beautiful girl i could fate ive been together with her for months more then half a year, we even got promise rings for eachother. But lately ive been talking to my ex and turns out she still into me and i feel the same way about her too... My girlfriend has a bestfriend and the shit they do they might as well be fucking on the side and im cool with that, would it be wrong that i do the same but instead with my ex?

Do dafuq you want fck your ex if you have feelings for her left and play it cool.

Well I'm not completely sure what your morales are, but personally I would hate it if someone was seeing someone else while we were dating. Loyalty and trust are very important factors to a relationship. If neither of those exist, which from what you've said they don't, then the relationship isn't a healthy one.