IF you go back to her you're setting the bar very low of what you expect from her and how you expect to be treated.
Sarah has lots of issues, she has very weak will, she has issues controlling her alcohol consumption and obviously controlimg her impulses. This woman sounds unhealthy mentally and emotionally and she just sent you an emotional sucker punch (cheating on you at the first opportunity that she's away from you) and you're considering positioning yourself for another blow.
Sarah doesn't even love herself, Friendly Guy. Coming from another person in their 20s, I say cut your losses and make this a better year than it started out to be. It's unfortunate that Sarah treated you so badly but you DO NOT deserve to be cheated on. There is a woman out there that will love you deeply, who is emotionally and mentally healthy and she will be faithful to you.