I can understand what you are going through, and sorry it has happened in the way it did.
But.. maybe he was ok with the relationship. Maybe he was being honest.. maybe he wasn't even fooling himself. It might well be that at some point he started to want his own primary.. or only..
You have known/loved and was friends with him for 18 years. Could he have carried the lie that long? I always try to assume the best in the people I love.. doesn't change the "what happened part" but it might change what could be in the future part. Resentments suck the wind out of life.
I am not trying to belittle your loss, thats tough after that many years. In fact, I couldn't imagine it on that level. Heal up..
best of luck on the healing.