This is so, so common. But - as PPs have said - one of the greatest taboos.
I’m an only child and looking like I’ll have an only myself. Well before I had him (shortly after I had met my now husband, actually) we went to visit my mum’s friend and her family in the country to which they’d emigrated. For purposes of this story, ill call friend’s son “S”.
We were all out having dinner one night and my mum’s pal said to my mum “Do you remember how you had just had Piglet and I had had S and you said to me ‘I really love Piglet but sometimes I wish I could leave her somewhere and just walk away’?”
Even then I knew that wasn’t an unreasonable thing to think and I can imagine I was probably a demanding child. I hadn’t had kids at that stage, but somehow totally understood it. My mum denied ever having said it and was at pains to tell me she didn’t feel that way. But I totally understand mothers can feel that way and it’s ok. She was and is a great mother - caring and empathetic.
I love my son and he’s fantastic. But the reality is that his vocab and communication with me is still pretty limited as he’s only 20 months and I can only really get on with things I need to do when he is asleep or my husband takes him out. It is so incredibly intense and the biggest life change I have ever experienced. I had PND too - in large part, exacerbated by the pandemic.
For some, parenthood is the making of them. But it you were your own person beforehand and had hobbies and a career and so on, it’s no wonder it’s a shock.
You are not alone, OP and anonymous fora like this are important so we can all know that and share similar experiences of this.