So I have 2 children to a previous relationship, first was in the first month of ttc, second was 3 months after the first was born

My partner had a child to his previous relationship which was also within the first month of ttc with her.

We are together now and have been ttc for over 8 months now, just feel like something is wrong with me as I have fallen pregnant so fast with the previous relationship.

I'm not sure if it's something to do with me or him? Who would it most likely be? Or are we just destined to not have a child together?

Also add that his child is 5 and my 2 are 4 and 3 so it has not been extremely long since we both successfully had children previously