My girlfriend is pregnant. She's happy about it, I'm not

It was just casual dating for a few months, and we've only been intimate a few times. She isn't on any birth control, and she made it clear she wasn't going to get on any so if I wanted to be intimate with her it was going to be on my end. I was stupid and didn't use protection and now she's pregnant.

She's only about 4 weeks, caught it during her routine blood work at the doctor's office. She's surprisingly optimistic about this, I am not. She's already thinking ahead and planning about putting parts of her paycheck aside to prepare for the baby, and has already started to eat a bit healthier.

I don't think I want this. She has two kids from a previous relationship that I like enough, but I don't think I can deal with a baby and toddler stage. She keeps telling me that it'll all be okay, that "everything happens for a reason and the universe had a plan for this" but I'm getting real tired of that hippie bullshit. I just wish I could go back in time and stop this whole thing from happening.

How do I talk with her about this? She's already so happy and attached to this thing but I just feel panic whenever I think about this thing being born. I need advice quickly!