I was going to start my own thread about this today! 3.5y old and 4 months old here. Also both good sleepers generally and yet I still find it tough.
My toddler practically never woke at night until DC2 was born so I guess it's to do with that and it feels like some sort of cruel joke! I can deal with the baby waking a lot better somehow.
Last night baby slept through till 4.30am which would have given me the most amazing block of sleep since they were born, but no, toddler decided to wake at 3am and not go back to sleep until 5am.
The fact baby was peacefully asleep kind of made it worse, I felt sort of resentful.
I have mild anxiety and sleep is a major trigger so once the whole house was asleep again I just lay there awake and anxious...
DH usually deals with toddler wakings as a rule but because there are now so many and he works full time I'm wondering if it should now be my job too so I'm scared I'll have to do it all soon. He's never dealt with baby wake ups though as I exclusively BF so he has had plenty of full nights in a separate room in the last 4 months, when I have had none. So not really sure what's fair.
When they both wake in the night it makes me want to scream and run away. Then they'll have a good night and I feel on top of the world again. Urgh, good luck OP, know you're not alone!