As an only child, I have a lifelong resentment that people feel the need to 'fix my lonely status' by having siblings. I have quite a few only child friends and they are all smart, mature, independent individuals.
In my experience people often have two or more because there's an element of " Oh, could you only have one? How tragic..." underlying the perception of only child families. So the second child is seen as ' completing' the family in a chocolate box "Nothing to see here!" kind of way.
You were considering a second child when your marriage was in a better place than it is now. To continue with that would be madness.
My parents separated when I was six - as far as I can work out from talking to adults who were exchanged between two households as children's, most of them resented it, preferred one parent to the other, happily played mum off against dad, played favourites and ABSOLUTELY KNEW what their parents were doing when they started the "So, did you have a good time with your Dad?" Type conversations.
Plus if you divorce, you will both inevitably find new partners, and with that will come stepchildren. Yours and possibly theirs, given your age. Go on to the Step parents forum on MN and see how that works out a lot of the time.
You have one child who you both love and will do the best for. Please, for their sake, leave it at that.