Advice on cheating GF

Soooo I'm 30M and currently attached, or not soon.. Just last week I caught my gf entering a hotel (entirely coincidental as I saw her near a crowded mall and only followed her for 400m or so), when she told me she was staying late for work. I know I'm assuming she's cheating but I can strongly feel it as she has been sneaky around her phone recently and her entire mood and appetite for sex have totally dampened too. For context we have strong sex drives and enjoy the frequent and intimate times. I'm not lazy and keep my shape up for her, so can't say that I'm not putting in my fair share of effort.

I'm just rollercoaster-ing between angry and upset, and I wish I could do smth to get back at her before i break up with her. But I guess that would be petty I don't know.. any advice?