I agree with Judy. It might just be the first day and the extra work that is a shock to you.
I started a new one in January because all fall I was starting to feel bored with the toddlers and thought I could handle a baby to keep me busy
Then the baby came and I was like "WTH was I thinking? I can't do this!" and was kicking myself of rnot enjoying my easier time with the almost 3 year olds.
Now the baby transitioned and things are smoothing out.
Sure I am still counting down the next few months until the baby becomes around15-16 months and more self sufficient in terms of feeding themselves more and walking more etc but I am feeling more okay with it nmow.

However - if you can't do it then I think sooner is better. It takes a lot of insight to admit it snd know your limits. No sense being miserable and not providing your best self with the other kids etc