I really don't know anymore

I have a daughter from another relationship & I've find out that me & my partner r having baby (I'm 6weeks & 5 days) now my first daughter has been give my partner a really hard time for example everytime my partner says anything to her she starts crying so then he just cracks it & goes off I've try to explain that she's to young to understand what she doing she's 1and half in August)

He says he is nearlly at the point were he wants to leave but what I don't understand is that they got on so well when we first got together & I can't work out what has changed between them