Don't get stuck over-analyzing your life!
There are some things that you just can't plan for, and don't pressure yourself to have your life 100% figured out at 23. I won't say you can't do anything, or can do everything. That will have to be a balance that you find for yourself. You're obviously a smart chickie, so make yourself the promise that you'll figure it out on the way, and stop worrying to much about it.
Remember, just because you enjoy doing something, doesn't mean you have to make it a career. You are obviously a very achievement-oriented person (like someone I could look in the mirror and see).
At 29, I also enjoy writing and playing the piano, but right now, raising my four- and seven-year old, working full time, working on a Masters, and managing a household means that my plate is full. I still scribble snippets of story starters in the notebook in my purse, develop my writing through journaling my kids' antics, and play piano strictly for my own enjoyment, but I don't try to pressure myself into writing the next great American novel or play with the local symphony right now. Instead, I use those things as a way to relax from the chaos of everything else.
Instead of looking at your options as 100% yes or no questions right now, look from a different perspective.
If you enjoy writing go for it! What's stopping you from submitting now? Maybe you'll surprise yourself. If not, keep working on it and enjoy the process of developing your voice.
If you enjoy science, but can't see yourself dedicated to a full time research career, couldn't you use your education to teach part-time at a local or online college? Remember, once you have the education, you can always fall back on it if you're worried about feeling financially dependent.
You've obviously figured out how to succeed if you've done so much by 23, so don't worry too much about failure in the future.
Having kids doesn't take away your life, they enrich it. Play by ear, enjoy every day, have fun with the opportunities you have now, and take what comes as it comes!