Stop worrying.
I know that's a pointless thing to tell a fifteen year old - I don't think I stopped worrying about things I couldn't control until I was twenty-two.
Then there's those of us who continually worry about things we can't control well into adulthood. (Raising kids does tend to remind you about how much control you wish you had vs. how little you actually do.)
Anyway, to the OP, as others have said, write the book. Don't think about marketing or how similar it is to what's out there or even your ability to accurately portray someone considerably older than you at this point. First books are about getting it done. It seems simple on the surface, but it's where many aspiring authors give up. Jump that hurdle first. There will be plenty more ahead of you and you'll deal with each as it comes up.
Oh, and as someone who's staring at the big 3-0 on my horizon... I promise, it's not as old as it seems when you're a teen. It has its perks, too. When I try really hard, I can actually get people to take me seriously instead of giving me that, "Oh, you're just a kid, what do you know?" look.
I love being a mom too. And running the house the way I want. Neither of those would've have worked out so well in my teen years. (Oh, and for whatever it's worth, I continued looking like a teenager well into my 20's. I actually asked my hair stylist to cut it so I looked
older.)
Anyways, best of luck and congrats on that first chapter!