I'm so sorry this happened. I totally understand the effect that depression can have on a person. I've suffered from severe clinical depression since I was a small child, so it's been no fun.
Was your wife supportive? Didn't she understand what you were going thru? Was there something she just didn't "get?" Is that why you turned to porn and the other woman?
It seems to me that she could forgive ONE "virtual" affair. Afterall, it wasn't in person -- correct? If you two had been meeting in person, that would be different. But from what I understand, you didn't, so I don't understand her unwillingness to forgive!

Has SHE never made a mistake? Is SHE blameless in the 'fall' in the marriage? I doubt it. I have a feeling that she has done some things to ruin this marriage too. And perhaps there's more to this story than we're hearing.
I'm really sorry you're hurting so badly. You might want to get some counseling to help you deal with this. Perhaps even grief counseling would help. You can contact a grief counselor thru Hospice -- they'll be happy to talk to you. God bless you and PLEASE take care! If you'd like to talk more, feel free to private message me any time. Take care of yourself and God bless. Hugs, Lee