Kaji's my first dog, and I got him a month before my 24th birthday. It is such a huge adjustment. Although, looking back, almost 3 years later, I don't understand what my apprehension was all about.
OP you'll get there too.
Although, if your feelings don't change within a week or two it wouldn't be a bad idea to re-evaluate your decision. You're feelings are there for a reason. Right now, it could just be an overwhelming feeling because having a puppy is just so new and foreign. After a week, you kind of get a sense of what having a pup around is all about. Including the joy and the not so joyful moments. Kaji and I still struggle with his anxiety. While it's about a million times better, there are still moments when even I, his beloved momma, scares him and he pees out of fright. Just re-washed all my bed sheets and comforter yesterday.
Even while grumbling about having to wash everything again, Kaji managed to sneak me that "I LOVE YOU" look and all was forgiven. I swear, this 7 lb ball of fluff owns me!!!