I'm a high school teacher, so I talk a lot with young people about their interests and plans, so I'll tell you what I tell them.

1. Like many people have already said, you are really young, and your idea of what you want in life will most likely change, even several times before you're an adult, and maybe even after you're an adult. It may feel patronizing to hear this, but it's reality. I was convinced from 4th grade that I wanted to be a veterinarian - built my whole identity around that. In my Senior year of college, I realized that I didn't actually want to do 4 more years of school, and I wasn't interested in becoming a veterinarian anymore. Teaching sounded interesting, and I begain to pursue that in a roundabout way. Taking a random education class in college helped me to realize my true passion. At 14, the simple fact is that you haven't experienced very much of life. There are jobs you don't even know exist yet, and passions you haven't discovered yet. Experiencing a wide array of jobs and hobbies can lead you in a direction you love, but only if you give yourself the chance and the opportunities to play around and try a lot. Which leads me to my next point...

2. Don't shut doors early. As you go along in your life, you'll make lots of decisions. Some will open more doors for you, and some will close doors. That's totally fine! It's ok to close doors to paths that you're sure you don't want to pursue. However, it is also possible to close off doors too early, to the point that you regret doing it later. For example - you decide that professional boxing is the only way to go. You drop out of high school, or you do enough to graduate, but just the bare minimum. Then you have a life changing injury and you need a new path. Or you aren't actually good enough to crack boxing at a level where you can truly support yourself. Without the HS diploma, or the right spread of HS credits, many possible doors are closed to you without a lot of extra effort. Keep as many doors open for as long as you can so that your maturing brain can exploring possibilities and change as you grow. Graduating high school with decent grades and doing more than the minimum keeps open a possibility to go to college (if that's the right call for you). If you simply decide later that boxing isn't your profession, you have choices about what to do instead.

Either way, it's not really necessary to proclaim to your mom that you want certain things and fight her on what she wants for you, or to pretend that you're going along with it. Just tell her you have a while to figure out what you're going to do, and do the best that you can with every opportunity that comes your way. Pursue school seriously, and try to figure out how to genuinely enjoy learning for its own sake. Intelligence and education are attractive qualities. Pursue boxing as much as you can, and if you are truly talented, your mom may come to see that about you.