DH and I had been together for nearly ten years (married for five) when we had our first DC. Adjusting to motherhood was a million times harder than I ever thought it would be. I cannot imagine how I'd have gotten through the first couple of years having to rely on someone I hardly knew for emotional and practical support when I was at my most vulnerable. Sleep deprivation brings out the worst in most people and it's a lot easier to forgive the bad moods, the things said in the heat of the moment etc when you know someone so well (because of your shared history, the time you've invested and the depth of your feelings for one another) that you can be certain this isn't really them, it's just exhaustion and it's temporary.
Do you even live together? At the very least, find out if you still like one another living under the same roof before you do something as drastic as TTC.