I'll be honest. I didn't read the full post but I stopped at where he wrote that your relationship is going to its end. Honestly, do you want to stay with someone who clearly admits to someone else (plus it's someone for who he has feelings) that your relationship is almost over? That sounds very lame, rude and disrespectfull towards you because it's YOU with who he should talk about things like these.

I don't know what are you going to do next but you should really confront him about it or talk how is he seeing your relationship.

Plus they are meeting behind your back, that's also unacceptable.