HC - I just feel so sad for you. I am so sorry for my "take responsibility" advice. There is no way you are responsible for his behavior.
I am also seeing your silver lining. I'm glad he finally confessed himself to you, so that you can now know you were not dealing with just the usual detached man and usual marital problems. This one is bigger and beyond most.
If you go over to the infidelity threads, maybe you will get more support from others who have been through anything similar to yours.
Hang in there, sister. I hope you have a good support group around you.