It worth doing math as mediocre student?

Hello i want to ask some advice:

I'm not so good at math, as i failed many classes becoz of math but still i end grinding and get out from my high school with good grade. Now i want to do mathematics, but i don't know if its worth doing something like that:

can someone who isn't a genius at math can do phd and do mathematical research if he work hard enought? Or it is gatekeeped?