I am as confused as it gets?

My ex gf cheated on me with some slime ball guy that is 9 years older than her, we had a perfect relationship and she had absolutely no reason to cheat. She denied cheating on me ually to this day but I know for a fact she did. She tried to tell me she fell in love with him, please. Now she misses work 4 out of five days a week, all her bills are behind, she's abusing drugs, and is now talking/cheating with another guy she works with on her new bf, what gives? What the heck happened to this girl that used to tell everyone in her family I was her future husband????