Stepping back a bit...
Many a relationship is rocky when a child is 18 mo. It’s a challenging time for any relationship. You say his selfishness & laziness is a key issue but you have both been trying to do better, & things have improved a bit.
You are not entirely settled in this relationship & consider ending the relationship to live as a single parent family. I assume that as you have not explicitly mentioned this that no other extra-curricular people are involved (!).
I reckon most women with an 18mo feel this way at times. The thought of a life without a pesky adult getting in the way is appealing. BUT would being a single parent who would like to have another child within a couple of years be so straightforward/easier/manageable or tolerable compared to your family life now? As you say, you don’t know, which is a sensible perspective.
So, how are you in other ways? In yourself? Might you be mildly depressed or have low key PND? I suggest you check in with your GP for a general MOT & share some of the challenges you are feeling. Are you getting enough support as a new mum? Support for him as a new dad? Did you both realise how much changes when children come along?
If you both feel you have challenges between you that you cannot resolve together, then book in with Relate for a preliminary consultation. Lead times for first appts after Chrismas holidays are long, but you may find that getting a booking in place gives you both space to breathe & start talking at a different level.
Deciding & negotiating when to have another child can be tricky & fraught. Remember that sometimes things do not go to plan & it can far long to conceive than wished. So many an odd/happy accident takes place whilst these dilemmas linger, & Lo, a new baby cometh 9 months later.
My point being that both of you know how babies are made, intellectually you are both wrangling & negotiating when next to get going on this, but at another level you have both decided at another level to take a gamble & do the deed anyway. Give fate a mighty legover.
How close you reckon you were to ovulation, unless you are a perfect ovulation tracker, you cannot actually know you were fertile AND he did not ask. Yes, you could be pregnant & no, you could not be pregnant. More stick peeing required.
So stop beating yourself up. You are not bad or terrible. Yes, you are both taking some risks. Are there other areas in your relationship where you both talk & talk round matters in an adult informed intelligent way & then opt to do what you really fancied doing anyway? Much like many couples on the planet.
Seriously, if there problems between you, go to Relate. You both have a duty to your child to do whatever you reasonably can to be a happy well-functioning family. If you cannot resolve your differences, Relate can support you both to make a decision to separate.