How can a long-term boyfriend move on so quickly and just stop talking to me so easily?

We dated a year and were very serious. He constantly told me he was going to marry me. I'm 20 and he's 24. He tried so hard in the beginning to keep me/make sure I was going to stay with him. He always said how he was so afraid to lose me and that I could get any guy I wanted to, so he was always telling me how lucky he was and blah blah. Before our break up, I noticed him "checking out" It was like he just became so careless and distant around me. He stopped making time for me, stopped putting in effort and stopped making me feel wanted. I messaged him how I felt cause he canceled plans again and it made him mad. 3 days go by and I don't hear anything from him so I say "why are we fighting" he said "It's not working out." I said "what do you want to do?" and he says "It's like no matter what I do you still aren't happy so I don't know" and I told him how I dont want to break up over such an easy fix and go on about how I love him... He just ignores me and doesn't reply. He never directly said he wanted to break up which left me super confused. I did no contact and after a month tried talking to him/calling him but he didn't respond or answer. He just won't talk to me. I don't get it. 2 months later I hear he's dating his co-worker (who he talked about while we were dating..) and she is such a downgrade honestly. Him and I had so much chemistry that I don't know how anyone could top it and I really did the world for him. I just want to know how this has been so easy for him, why he won't talk to me at all, and why he's already dating someone? It is literally killing me and I don't understand how he can go about life like we never happened. Does it mean he never loved me and never thinks about me?
How can a long-term boyfriend move on so quickly and just stop talking to me so easily?
Add Opinion