Because I get the feeling he was lying before.
And it'll likely be easier for you if you start thinking that way.
At least he was upfront enough with you to tell you it would not be forever, and he did you a kindness to end it early.
You need counseling. Seriously. You need to talk to an impartial professional about how this is making you feel. And, in time, talk about why you spent 3 years tilting at windmills with someone you could never be with. Why you settled for being second best and part-time. Why you didn't or couldn't or were afraid of going out into the dating world and finding someone who could be yours 24/7.