It is very difficult to help someone in depression. A depressed person has to recognize that something is wrong with him/her. Do not blame yourself because of course he would have gone through a grieving process when his mother died. But his seeking the company of another woman while married is not your fault, rather, it demonstrates his poor coping skills.
And when I say OW is getting a "poorer version of your h," I didn't mean in financial terms! I meant a poorer man in emotional and moral terms. The money, to me, has nothing to do with him being a better version of himself.
If you do the reading on affairs, he feels so bad about himself/things that he turned to OW to make him feel better (a band-aid). Because he is so broken emotionally (plus he is married!) he can only attract a certain person. That pool of women is not good. That's why they all affair down.
I am also 41. So I understand how you feel. As for the dating, it's tricky. If we aren't fully healed we probably won't be attracting the best quality person. And if we do meet some wonderful person, we may not be ready as we ourselves are still healing.
The question is how do you begin to rebuild yourself in a healthy way?
Me 41, H 47, M 15 yrs, S11, S13 BD 1: 11/4/14 we work on it; really I pretzel myself BD 2: 3/31/15 H goes down to "dorm room" 8/15: H back to MBR 10/15: H back in dorm room 1/18: H files, now divorced