Help me figure out myself

I'll make this quick. I haven't studied properly for so long. It's like my Brian is mushy from being so untrained. But I know I'm not dumb, I think I'm smarter than average. But since my Brian isn't trained for years now, people with lesser talent are getting better grades then me.

How do I fix this? If I want to study a math chapter, how do I master it? Because it seems impossible to solve questions different from my book. I have all math chapters to do, and only about 2 months left. How do I train my Brian to be better? How long will it take? Something happened recently which just made me realize what a wasted potential I am. I want to make things right again.

Also, is it possible for someone to become naturally smart? Even if they're not born with it? Like habbits or something?

My life is absolutely shit right now, and getting good grades would make it my ideal life. Such a big change and only with grades. I really...need grades to be able to live.