it is without a doubt, 100% cheating. It matters very little if he met her but with him saying he was in love with her is beyond imagining. This is jumping in with both feet. It matters very little that it was online, email, chat, phone and all that you can carry on and get very emotionally attached before even meeting in person. He knew what he was doing - he did it in the "dark" in other words at times when you would not know. He hid it, he kept up the front that things were normal on the surface and lived a double life.

I cannot give advice as to what you should do but I can offer this.

you are NOT in any way feeling anything unreasonable, you are not obsessed. Everything that has happened has given you every right to be distrusting of him and "paranoid" of his every move. I don't blame you one bit because I've been there. I don't know if the trust can ever be mended. I would commend you on the ability to get over it if you did. seriously.