Becoming a professional historian is not impossible. In the US, over half of the professional historians work in the public sector (museums, archives, preservation, corporate, etc.), so don't be quick to rule out alternatives. I always told my students to study what they liked, that way they would have a good experience and they would receive the best grades. I also told them to be flexible and to consider all options. If you're in the US, you can look forward to a university experience that will broaden your horizon, and it is likely that you will change your major (the average is about one major change per student). If you're in the UK, changing direction is a little more difficult, so you need to be more set on your path. But at your age, don't be afraid to dream.

I have known many young people who thought they wanted to be historians, but who changed directions (and many others who changed to become historians). The one thing I would say is that jobs are scarce. If you are not prepared to be at the top of your class, it is likely that you won't find yourself succeeding as you follow the narrow path through graduate school and to a tenure-track professorship. In a few years, you'll need to ask yourself if you truly have the fire in your belly to drive yourself to the highest level of success. That may very well be the case, and I always encourage optimism. The fact that you are asking this question at 15 suggests you are already ahead of many of your peers, so don't be quick to imagine only failure.

At the same time, you may find that you really like history, but it is not the best path toward a financially successful and/or rewarding career and life. A lot of people consume history in a lot of different ways. I funded archaeological excavations for two decades, and most of the heavy lifting - literally - was done by volunteers who loved history but had other occupations. There are so many ways now through volunteerism, continuing education, and this really cool reddit site called /r/AskHistorians - to name a few options - where you can enjoy history. So even if it doesn't turn out to be your occupation, it can be your vocation.

Good luck on thinking this through, and best wishes to you for a wonderful future. Whatever you do, try to make the world a better place. My tired old generation is counting on you!

I'm a graduate student in a top US history program, so I'm pretty well acquainted with these kinds of issues:

1. Becoming a professor is difficult and fairly rare. There are hundreds of history PhDs who graduate each year, most of whom have no realistic chance at a tenure-track academic job (i.e. where you get paid to research and teach and have good job security). Those jobs generally go to people with PhDs from top universities (Harvard/Princeton/Yale/Berkeley/etc.) with good publication records (articles in top journals, books, etc.). That's not to say that it's impossible, but you should know what it's like going in. The academic job market is rough.

2. You're way too young to really worry about this. Right now, focus on getting into a good college and getting top marks in that college. I thought I wanted to be a doctor in high school, and here I am 5 years after I graduated HS in a history graduate program. Your interests can and likely will change.

3. There are many, many jobs in other areas. Museums, archives, and libraries are just three of the areas where academic historians work outside of a standard professorship.

Not to be too cynical about this, but very few professors make differences in people's lives. For the one or two I had who did, I've probably had 15 who didn't.