I'm so sorry that you are dealing with this. It feels awful to feel as though you have been replaced.
A common saying around here is "more will be revealed". So far, that has been true for me and my STBXAH. Just wait, the 'real' alcoholic him will come shining through, no matter how much he has tried to cover it up. After 13 years with my AH, he decided to get a girlfriend and lead a whole other life with her for the past year. He pretended to other people that me and his children didn't even exist even though he was calling us daily and coming home when he had days off. I know the sting of being cast aside, but it is true, she did you a favor.
Be kind to yourself. Sending you hugs and strength,
Confetti