It's no use being stubborn.

Want to know why?
Let's say you have a relative in school. This relative is extraordinarily bright, and you think he can do anything he sets his mind to. He is in every single challenging class there is, and there is no way he can fail.
Until he does.
When he tells you he thinks he can't handle one of the classes anymore, you say, "Don't be stupid! You're so good at it. So you failed. Why?! Just try harder! You can do it!" Truth is, he hasn't exactly been trying his hardest, so he nods grimly and decides to go for it again.
Yet again, he's not doing well in that same class. You don't see him trying too hard, but he tells you he stresses out and does a lot of work; you just don't see it since you're not in school. After all, you're not attached at the hip or the same age or anything; you're already out of school, and you lived it very differently. You just wish that you had taken all the challenging courses because it would have made your life path better, so you advise him to do so. He complies again.
Three years later, he's taking the continuations of those same courses. He still can't do well in that one he always complains about. You tell him to just try, for crying out loud! After arguing, he grumbles and goes back to it.
The next week, you go over to visit him and he tells you that you have to leave; he's been studying all day for this test and needs to finally do well. Maybe this try will be different; maybe he'll do better. You beg him to stay; you're two hours away from home, if you could just sit and watch TV across the house without bothering him for a little bit, you'd be okay. He obliges. You keep sneaking peeks at him the entire time you're there; he's really killing himself over this test. When you finally leave, you feel so happy that you've finally seen him trying and are sure he will succeed.
A few days later, he calls you, fuming. "I completely bombed that last test," he says. "I just don't know why. I studied for hours. I swear, those tests are impossibly hard."
And then it hits you.
Maybe he just isn't suited to take that class.
You tell him this, and in an angry rage he says, "Well, DUHHH. That's what I've been trying to tell you for the past three years!"
And you didn't listen and kept encouraging him to do the impossible.

You know, it's great to encourage people, but when they keep failing, you just have to let it be and have them take a step down, because sometimes, they're just not ready.
Take this to heart before you learn it the hard way.