Hi op, I'm sorry to read your posts and hope that today is a little better than yesterday.
I was in a similar position a few years ago. My e husband had become distant, unkind and, I guess checked out all of a sudden. I asked him what was wrong, thinking the worst would be a mid-life crisis or depression. He told me he wasn't happy but couldn't say why. I spent the next couple of weeks desperately trying to do better, be better, make everything ok, until I found evidence that he was having an affair. His bag was packed when he got home from work that day.
It was horrendous. Telling our 2 dc (10 and 11 at the time) was the worse experience of my life. I felt broken and I was bereft but most importantly, I was a mum and their dad had just let them down massively so I wasn't going to.
I cried buckets when they weren't around or in bed but I went into survival mode when they were and got things done. I saw a solicitor to find out where I stood legally and I went for counselling, which gave me the space to talk freely to someone who was there to listen and didn't know either of us. My friends and family were great but they all had their own lives.
As others have said, he will have had more time to get his head around this - he will have checked out some time ago, whereas you had it thrust upon you and had to deal with it.
He is not the man you knew and you have to try and separate who he was (or who you thought he was) for who he is now. I often wished he had died (I didn't really for our DC's sake but I did for me) as death is awful but it's final. Having to see the person you love every day and heal at the same time is cruel.
You are worth something to people though op - your son needs you more than you know and I promise this will get easier. I still feel sad sometimes but I have been in a relationship for 3 years now, after dipping my toes into dating for some company on my child-free weekends (I agree, that is tough when your friends are all in relationships/married). My ex split with the other woman after a year or so because they were too different, and has just split with another woman after a couple of years. He wanted freedom and he has it but he doesn't seem happy. I never wanted freedom but I have embraced it.
Keep on keeping on and gradually, you won't feel quite so raw and hopeless. It's a long journey but you are worth fighting it.