Regretting adopting a 2nd cat

Hi all, I adopted a new 3.5 month old kitten two days ago. I realize it’s only been a few short days, but I’m really regretting this decision. I have a resident cat who is almost 2 and who I thought could potentially enjoy having a friend. She is a shy, somewhat anxious cat. When I first adopted her, the adoption agency said she would do fine as an only cat. That’s definitely been the case- she’s settled into my house and life perfectly and we have a very special bond. She was originally at a cat cafe when we adopted her and we could tell she was pretty miserable there, always hiding and not really interacting with the others. Still, I want her to have the best life and thought it might do her good to have a buddy for when I’m working or have to go out of town. The new kitten in my mind is really more for my existing cat than for me.

Now I’m really questioning why on earth I thought that would be a good idea 🙃 I’m following the slow introduction method, but did allow my resident cat to smell and see the kitten while he was still in his carrier. Resident cat was growling, hissing, and overall NOT happy. I know this is normal behavior but I’ve never once seen her hiss or growl so this was not how I expected things to go. We gave new kitten his own space in a separate room with all the things he would need. However, he was so scared by our resident cat that he’s gone under a piece of furniture and will not come out. He’s been under there for over 48 hours at this point and has not touched food or water.

Resident cat is actively avoiding the door of the room the kitten is in but is otherwise acting mostly like herself, although she definitely has been crying more than usual.

I’m an anxious person and recognize I may be overreacting. But my gut is telling me I shouldn’t have adopted a second cat, when things were going so great with just one. I’m thinking back to when we first adopted her and wondering if “would do fine as an only cat” really means “should be an only cat.” Did I make a huge mistake?