It's almost impossible for me to focus on anything other than art. iin?

Well sooner or later you will be finished with mandatory school and you can focus on your passion.

I've always envied people who knew what they wanted to do with their life from the time they were very young. Many never do figure out what brings them joy, and some people know but they can't devote their lives to it because they're unable to make a living at it.

Getting through school is a matter of serving your time and jumping through the minimum number of hoops necessary to get you where you want to be at the end of it. If someone is determined to get a degree in a particular subject because they know they want a career in that field, then it makes sense for them to focus on the academic side of things and they're motivated to do so. If someone knows they want to be a self-employed potter, then it's hard for them to be motivated to get good marks in biology, calculus and English composition. Whether it's good for them in some abstract sense to be exposed to those subjects is another question, but they aren't directly relevant to their chosen work.

So where do you want to end up when you finish school? If getting good grades in non-art topics is necessary for you to get there, then you need to understand those classes are a means to an end. Grit your teeth, keep your eye on your ultimate goal, and jump through the hoops. It may well seem like a waste of your time and the teachers', but that's the system you're in.

I assume you're in secondary education (high school or equivalent) and you're older than the minimum school-leaving age where you live. If you're so good at art that you don't need to complete secondary level education to earn enough to have a standard of living you find acceptable, then think about whether you need to stay in school. If you can already make a decent living doing your thing, then school is a waste of time you could more usefully spend developing your artistic vision.

However, if you're not so good that your art is saleable at prices such that you could already have a decent income if you devoted yourself to it full-time, then I suggest you accept that the wise decision would be to keep your options open by buckling down and forcing yourself to complete secondary school with at least reasonable grades.

There are plenty of career choices in art and not all involve becoming an artist. You can work for auction houses, galleries, museums, etc... As a researcher, historian, curator, art restorer.
How old are you?

you may have adhd? I feel the same way and I have it.

Go talk to a counsellor, high five to your girlfriend.