You don't need to know what you want to do. You're allowed to change.
It is good to have a rough idea, just like this, mostly so you can guide yourself to certain skills. Drawing is a skill. Drawing only manga is a more limited skill that may be less marketable resulting in less likelihood to do something like what melindamusil's sister did. That is, however, not the only way to do things.
Writing is also a skill. Knowing a thing or two about story structure is a related, but separate skill. Ability to imagine worlds is a skill, too, but it's also something far more inherent to the human condition so, like any skill, is best employed in tandem with a couple of others for the purposes of entertainment (authorship and other creative industries) or service (marketing, advertising, production, design, etc.).
Once you've practiced these things you can bounce between relevant fields fairly easily. Practicing isn't necessarily enjoyable but it doesn't have to be boring. It will be sometimes. Pro hockey players have to do bag skates. But they find some enjoyment in the routine of it, live for the moment they get to scrimmage (or actually play hockey), and work hard the rest of the time. It is possible to enjoy hard work, and to take pleasure in the sense of accomplishment you got by moving forward... or by hacking away at a block even if it doesn't move a great deal.
I don't know that there is any advice that will make life a constant stream of blissful ease. The bad news is that you won't always get advice. The good news is that, as you practice and accumulate that wonderful intangible "experience," you increasingly won't need it, at which point a whole other challenge arises where it is all of a sudden very difficult to hear feedback and act on it in a way that allows one to get better.
What everyone is saying here is that the lumps you take actually make the good parts worth it.
A word of caution: every human being I have ever met is capable of imagining worlds in his or her head. It's actually kind of essential, in a psychological sort of way, to how we communicate on a day-to-day level. Some people aren't as good at realizing their worlds in words or art and prefer instead politics. Some people imagine worlds that frankly scare me and I usually hope those people write dystopian horror novels instead of running for public office. The question we have to ask is how am I capable of expressing these things that excite me. Some people imagine worlds that have everything to do with competitive knitting (is that even a thing? I really, really hope it is) but I can't get excited by that. I am likely not their audience. If we want to share our worlds with others, including those others who might be curious or kind enough to give us money for the privilege of being entertained, we have to do the hard work of finding a way to communicate what we imagine in a way that is entertaining, skillful, and interesting to a given audience. That is the fundamental difference, at least in my opinion, between craft and imagination. You need both.
You may also find you need discipline to achieve a level of craft that satisfies your own and others' expectations. One of the toughest things about being someone who makes things is overcoming your own standards for consumption. If I love movies dearly and I try to make one, my first movie will likely be a steaming pile of crap. It will be riddled with visual mistakes, technical errors, clunky writing, bad acting, the lot. I will hate it because I am very good at watching movies--much better than I am at making them--and I expect certain standards, even subconsciously. The difference is whether I then pick myself up and make another movie, which will also be crap but likely slightly better crap than the previous attempt, or whether I throw in the towel and find a spot on the competitive knitting circuit.
Nobody said this was easy. My first novel is in the trunk because I wrote and revised myself into a fundamental plot black hole and it was more practical to start something else than re-do the entire storyline. I remember reading your
Psyhero piece in one of my first ever crits on this board. This is what I wrote in my crit then:
I also started writing and self-editing around your age. I also tended to have great world ideas and some execution problems.
The execution got better naturally. I kept reading. I kept learning. Went and did more school (am still doing school...) and I now have to rewrite any early MSs because my writing voice is so different. Line-by-line writing gets easier with experience, so just keep doing it.
When I started, I wrote "EXPAND" in big letters next to almost every line. I had the same reaction to my first drafts as I did reading yours. There was line to hook me (often dialogue) and then no expansion. No real story, just characters talking. I didn't learn about "Show vs. Tell" until later, but I needed to slow the pace, flesh out scenes. I wrote the first section (about 40 pages), then rewrote and it expanded on its own.
I can see that happening with yours, but go even further. Close the original doc entirely. Your mind will remember what it said but add new things. Then cross-reference the original and see what you missed on rewrite.
What may eventually happen is you swing too far the other way. Paragraphs become bloated and overexpanded. Then you cut. Rigourously.
Now, that was six months ago so you've probably improved. But if you're not letting yourself experiment you won't improve. If you're not putting words on paper--and it has to be more than occasional; exactly how much is up to you--you're not going to get better and you won't start to drift into other new and exciting problems.
If you're not trying it makes it hard for us to give you advice. That said, you can feel free to PM me any time and I promise I will eventually respond to it as best I can when I get a moment away from the sixteen other projects in need of urgent attention. Or, you can post a question here or in SYW where everyone can chip in with different ideas when they are around. You will get weird ideas, conflicting advice, harsh critique--but you will learn and continue to learn. Being young, you're not in a bad spot, really, as long as you keep at it.
Best of luck!