Might sound weird but I have become obsessed with pies (proper pastry ones with a lid). During the first 3 months I probably had five a week. They are a whole meal in one comforting parcel and keep their heat incredibly well for the days when it takes two hours to get to it. I once left one in the oven for upwards of 90 minutes and it was still delicious. If your oven has a timer delay then even better. I put the pie in first thing (i.e. sometimes 4am) and regardless of whatever shitshow the day has become, the oven cheerfully heats it up at midday and keeps it warm until I can run over and shove it in my face.
Frozen pain au chocolat. Whatever the night has held, whacking one of these in the oven for 15 minutes can make it all better again.
Thermos mug is a godsend, make a hot drink at bedtime and it'll still be hot at 3am. I also went through a phase of pre-loading all the mugs in our house with teabags.
Wireless headphones are great, sometimes when I know I'm in for an evening of bouncing a grizzly baby I just put on a podcast or Beyonce on an endless loop.
Whiteboard calendar. My memory is awful at the moment (DD is 5 months) plus it can be really easy to lose track of what day of the week it is. I got two magnetic whiteboard calendar things to go on the fridge (think they were a tenner for two on Amazon), one showing the month and one the week. It helps me keep stuff in perspective and space out visitors etc.
I made the mistake at the beginning of trying to frantically do loads of housework/admin/washing during naps, then the baby would wake up and I'd realise I had indeed done three loads of washing but I was still starving and needed a wee and now it was too late. So now I have a strict order of business: loo, cuppa, food, a few minutes dancing/crying/sitting down (depending on the kind of day it's been) then everything else.
A soft apron or pair of dungarees with pockets is great for carrying around your phone, muslin, snack etc.
A big yoga ball is fab for those times when the baby has to be bounced to sleep and your knees are exhausted.
Also re getting out of the house: I spent the first couple of months either forcing myself out and overdoing it, or feeling trapped inside. So now I have a one-on-one-off policy of "activity day" vs "home day". I've never got on with the group activities so for me an activity day means a long walk in the park or a coffee with a friend. So I get out probably three times a week. I have to have home days in-between otherwise it's too exhausting, and the key for me has been to make the home days really super snuggly so that it feels like an active choice to stay in.