she got that promotion and is now ready to trade you in for a better model since you've not improved your life status like she just did. she did better and is now looking to be with someone that is at the right status level to match her current and newly improved situation.

ps i know it sucks but do not lie to yourself. this is gonna hurt no matter what but if you keep denying what you know happened it will just be a raw wound that keeps getting ripped out with each new bad detail. accept it for what it is, and try to move on emotionally and get her out of your life ASAP. easier said than done i know but there is 0% chance she didn't fuck him. if you told me 99% of affairs start in the work place i'd be suspicious about how it's not 100%. this is literally the most cookie cutter affair shit that all of us have been blindsided by the first time we learned love isn't forever. good luck my man. don't turn to substances and lean on your friends and family to keep you balanced.