You are struggling and that's very very understandable.
But please don't spend too much energy bemoaning your ex, however unfair it all is, as this is energy you can be redirecting to finding solutions, even if they are imperfect ones.
I think you've hit a wall with ex, and if you have funds you might need to get practical about other forms of respite, even if it's not the full rest you need atm. Like a mother's help, who may build up to being a much needed babysitter one day (I'm just suggesting this as an example, as additional nursery hours didn't seem an option).
You mentioned ex's extended family - would they be open to and suitable for the occasional Saturday of care (leaving ex out if it)? I appreciate that may be a long shot.
If it helps, when your DS is out of the toddler stage, you might regret it if you'd given up every weekend with him, and just left yourself with the drudgery of the weekdays.
Really really try and make the most of the weekends you do have free, focusing on what you need most. I know comparisons aren't always helpful when you're struggling, but 2 completely free weekends is more than coupled women get. One would hope they don't carry the full load during the week too, but let's face it, many do.
You really sound like you need to recharge your battery. How does it work with your annual leave when kid is at nursery? Can you take time off work that doesn't need to be spend with DS?
Wishing you luck and a chance to catch your breath.