Crying, and more crying

I just joined this group tonight, so bear with me if I say something stupid.
My live-in BF of nearly 13 years recently told me that he found someone new.  He continued to live with me and go out on "dates" with her.  I couldn't take it anymore.  I was miserable and he was totally blind to my pain.  He called me psycho.  He said he didn't want to hear "my shit".  I finally told him that one of us had to go.
I came home yesterday and he was packing his truck.  He told me he'd be back today to finish.  I cried all night.  Yep, he came back today and took the remainder of his stuff.  I have been crying all day.  He just walked past me like he couldn't see me.
How is possible to cry this much?  How is it possible for him to just not care?
I seriously don't understand.