This one is extremely cliché, I know, but it really does help!

The key is to stay positive. What you say affects your mindset. You should keep a growth mindset;
Instead of saying "I'm a failure"
you should say "I'm going to try harder so that I will do amazing things"

I know that being positive can be difficult at times, but it is the key to success. If you get a question wrong, instead of kicking yourself for making a mistake, try again and tell yourself that you are smart and can figure it out. I, personally, have trouble with this concept. But trust me, one you find a way to be positive, studying will be easier.

There is an anonymous quote that states:
"Study hard, do good, and the good life will follow."

That means that if you study well, do good (as in; be nice, good grades, etc.), and the remainder of your life will thank you for that. It will be good to you (it most cases anyways. That's not always a promise. Fate has a way of taking control)

But enough about that, right? You get the idea.