How do guys move on so fast?

ive been dating an living with my bf for almost 2 yrs we have a baby together he was there my whole pregnancy he nevr broke up with me but i broke up with him the otther night which they were good reasons tho nothing about cheating or lying but i guess i really needed to end it he needs time to grow up an be able to support his family but he acted like he didnt care cause he was like saying youll come back i no it blah blah blah and i told him i wouldnt this time which hes so sure i will come back but he seems okay an just fine im trying to act like i am to though but he supposbly loved an needed me just the other day an said hee wouldnt be able to live if he ever lost us but idk im confused now he acts likke he doesnt care an weve been together for almost two yrs we were barley ever apart an i mean barleyyy ever apart we did everything together im confused on what to do?