What Happened?

This has been a very trying couple of weeks for me.  More of my STBX's life is coming out and he is acting even worse towards me and our sons.  He is now known as driving drunk and hooking up with whatever has a vagina and shows him some interest.  He refused to sign the paternity paperwork acknowledging our youngest son is his because he wants to be properly served by the sheriff.  He has told our youngest son he does not have any money to do things with him, but he is at hotels every weekend and taking these "things" out to eat almost every night.  He barely calls our son and got mad at me because I cannot afford to pay for a family gym membership anymore. 
I was feeling so empowered, but this has taken a toll on me.  I am seeing that the man I would have given my life for, just flat out does not love or even care about me anymore.  I know I will never know the answer, but how do you just leave your family for a life of drinking, clubbing and loose girls?  You are 40 years old!!  My friends and his friends say that he is on the verge of hitting rock bottom, but what is his bottom?  It's like his family does not exists.  On top of it, no one is even talking to him about this absurd behavior he has taken on.  I found out a few days ago that he is not even being treated for "depression" anymore.  He just stopped going to his sessions and the doctors do not know if he is even taking the medication anymore. 
I am so tired of having to revisit this feeling of powerlessness.  I am feeling this way because I am allowing him to once again control my reactions and emotions.  He left and I have accepted that.  It is this extra stuff he is doing with his life that is getting to me.  I thought I was doing really good, but this hits me.  I understand that you cannot get over 16 years in 3 months and I am not going to punish myself for this small hiccup, but I am tired of this.  I wish that I could just go on and not care about anything that he is doing, but it is hard because his actions can affect our sons.  If he is going to fall, then I hope it is soon.  This is tiring.