Have you managed to speak to your GP yet? Ask nursery to drop his naps. It’s probably about time for him.
Sleep is your friend, and you will be better off getting to bed at the same time as him, and embracing the early mornings. Try for the next week to go to sleep at 8 with him. Lights out, snuggles in your bed. Get up with him at 6.
My biggest mistake with my first was faffing for hours of an evening trying to get him to sleep in his own bed, and again trying to get him back to sleep in the morning. Huge waste of time and energy getting frustrated. I now snuggle dc3 until they fall asleep. If that means I fall asleep too and miss an evening of Netflix, so be it!
If you can’t increase childcare, then use your day with dc to have a stupidly busy day out of the house. Pack a picnic and leave the house the same time as your partner. Picnics, swimming, mums groups, shopping, walks, do as much as possible. Being out will help you mentally.
Know you are not alone. I cut my mat leave short and increased my hours for the reason that I cannot cope being at home on my own. I’m less anxious, more productive and happy for it.
I also think you should take a few days off work and just chill, or do some self care (haircut, walk, swim, sleep, read). If you don’t have holiday to take, ask your GP if he could sign you off for a few days. Mental health is really important.