How petty is revenge?

My fiance got beyond pissed when I hung out with my ex (I made sure there was a chaperone in case of something stupid). I fully admit that in retrospect that was a very stupid dumb retarded idea. I was ready to roll over and take my lashes until this morning I find out I have been paying her steadily climbing phone bills; $75...$90....$125....$195?! That extra $120 is all from her texting another guy, a guy she broke up with me for 6 months ago then immediately came back (4 days). I pay for her place to live and most of her bills, but this is what pissed me off. We have had a rocky past but have been pretty solid for 5 months, then all of a sudden she gets cold, doesnt want to spend time around me (we live together), suddenly she's telling me I'm annoying. All of this leads full circle to me just wanting someone to hang out with (it happened to be the ex). I own the house, her car, our insurance... its all paid by me and in my name. I know this sounds dumb but I'm about to take her $250bday present back to recoup some of my phone payments and yes, it was already given to her. I'm so mad I just want to move out and stop paying, take the black mark to my credit, and watch the house repoed from under her preventing her from even getting her things out. How wrong is that?