Just seen above that your little one is 5 month and mostly taking 35 min naps. I would say the short naps are probably causing the overtiredness rather than the awake windows.
I’m right there with you, with the short naps. DS2 is currently 3 months and hit the regression early and is a monster for the 35 min nap! DS1 did the same. We’ve had several of the 40 min then 2 hr wake nights ourselves recently, mostly because DS1 hasn’t been at nursery so I haven’t been able to dedicate so much time to getting DS2 to nap.
Today, I really focussed on his sleep and he’s slept straight through both 40 mins and 2 hours so far. It really is overtiredness.
In my opinion (and I don’t claim to be an expert, just my experience and what I’ve read), first you need to clear the sleep debt. Spend a few days doing whatever you have to do to get baby more rested. Forget self settling, just get that baby to sleep. Once the cortisol levels have gone down, it should be easier to work with.
My current approach with DS2 is to find the balance between teaching self settling and stopping him getting overtired. So today
Nap 1 - in a sling to get at least 1 hour
Nap 2 - in the cot, focus on self settling to sleep. Actively helped resettle after 35 min wake and his did 2 hours overall (resettling is frustrating and doesn’t always work but worth trying)
Nap 3 - 35 min
Nap 4 - 35 min
Keep the last nap short but trying to get longer earlier in the day even if it means sling, walking in the buggy, driving in the car etc. Then pick other times to focus on self settling in their bed.
If you get a day of all short naps, make sure baby gets regular short naps. I remember DS1 having 5 a day at his worst point!
It’ll pass, but it sucks right now. Good luck