How do I tell my mum that I don't want my dog?

(First thing, we haven't actually adopted them yet.) I don't want my dog and I don't know how to tell her without sounding like an awful person.

We have one dog, I love her and my mum decide that we could get a second dog just for me so that I could do the things that she does with our first dog.

We found a dog and this dog was my literal dream dog. They were smaller than our current dog, were a breed that I really wanted and my mum was also on board with getting them, but their foster family decided to keep them as we were organizing a meet up with them.

Then my mum and I started to look at shelters, normal ones and ones that specifically had the type of dog I wanted. She wanted another collie or collie cross, I didn't.

We applied for a dog in one of the shelters that had the dog that I wanted and were shortlisted for them, but my mum organized a meeting with another shelter that mainly had collies. We went there, met two of them and my mum decide on one and basically needled me until I said yes about it. She also withdrew the application for the other dog.

We now have this dog in our house. She's lovely and very, very very shy, but I just can't deal with her.

She stresses me out so much, I've already had two breakdowns about her and we've only had her for about three days. Because she stresses me out so much I don't want to be around her. I was fine with her for the first day and a half, but after that I'm just not.

And I know I don't know her and she will come out of her shell eventually, but I just have this massive feeling that she isn't for me and I don't want to keep her and find out it's true.

My mum loves her, my dad is warming up to her and my sister likes her has well. And I just don't and I don't know what to do.

I know I probably sound selfish, but I just can't cope with her and I don't know how to tell anyone.