You will get through this.
I have read your others threads in relationships, and I think you should reread them, too.
I think (and certainly hope) that you are going to get angry.
You had one thread worrying why he was off sex, and what you could do to keep the relationship alone. There was the withdrawal, the refusal to answer questions, the controlling behaviour re your right to go out or get a job, and the leaving you with no explanation. Oh, and the drugs.
Really, he is a prize shit, and nothing you could ever have done would have helped here- he is basically self centred, addicted, uncaring, and he has left you spending at leat 18 months wondering what the hell was going on. He has ruined your time with your two year old, practically since she was born, as you have been so consumed with worry.
He will get his comeuppance, it is obvious he will. All those personality flaws will hit him in the face sooner or later, and older men having aaffairs with a twenty year old- well, the odds on that working out long term are piss poor.
I hope to god you never take him back. In a few months time, when you have your life back on track, a job hopefully, your kids, and a few more friends- which you will when you are not concentrating on his crappy issues full time- you will be glad of this day, believe me.