My first girlfriend/sexual partner used to do the "push away before cuming deal" and later in that relationship found out she did it because I wasn't very good and she got bored and just wanted to move on to penetration. I had a feeling that's what it was and when I asked her she came clean. After that she basically led me through a step by step process until I learned to make her cum. which I was thankful for, because it taught me two things first: always communicate with your partner about sex, and second how to actually be good at oral!
Anyways I'm not saying this is your situation, I was just relating my experience.
Anyways I'm not saying this is your situation, I was just relating my experience.