It is frustrating because he doesn't understand that most people have feelings, he usually makes fun of people and gets in trouble for swearing at younger kids at his school. I forgot to mention that he is eleven, but I guess what bothers me is that he treats all his ideas like a sacred text. I've taken writing courses for two years, and have been using the things that I learned to add to my scripts. But he doesn't have that much experience in writing. I sometimes cringe when I read our scripts because they might have poor grammar structure and other times the lines are just corny, and that goes for both of our scripts, the ones that I write and the ones he writes. I try to take criticism in a positive manner, but it definitely is hard not to want to insult someone else's writing when they insult your writing. There are times when his scripts lack story and the lines are just random talk, but I make sure never to say anything like "this is terrible" I usually try to be positive and say "this is good, but try to add to the lines and make the story more clear." So how should I take criticism and not lash out at him and his scripts when he tells me that my scripts aren't that good?
He is eleven?
I wondered exactly what the situation was. Your profile says you are older but the dynamics sounded like you and your cousin are very much younger.
It's certainly a good thing to think of such things and to be aware and try to help others, and I don't mean to dismiss any efforts to improve yourselves only because you are young, but if he is that young, he really may need to grow up a bit before he can act in a more mature way.
I think it's ok, and a good idea, to give him general critiques on his writing. He might take it better and get less defensive if you say "they taught in writing class that it's a better idea to do xyz" - that kind of thing. If you come across that he sees you saying your writing is better and his is bad, it sounds like he is probably becoming defensive and seeing it all as a contest between you two. And if you played together a lot when you were younger, he may just still be stuck in that practice of playing and trying to win or be the best.
And these days we have this HUGE culture of raising kids by making them think everything they do is wonderful, so they don't feel badly about themselves. But sometimes (often) that goes too far, and so some kids need to learn the truth of things. He might be affected by all of this and not ready to learn to be more objective. He may still have people telling him things (how great everything he does is) so that he can't accept criticism yet. I'm just guessing about all this part, but it happens a lot.
If you are anywhere near his age -
I really commend you for thinking about these things. It's good to start getting into a discipline of thinking about how we react to things, what that means for us spiritually, and how to think so that we can cooperate with God and begin to let Him shape us to be like Christ. Many people never take up that discipline, but I think it's VERY important. It is also one of the harder things you can do in your faith (maybe one reason why a lot of people never do it). So please, keep doing it! But don't be too hard on yourself and realize that it takes time to have our thoughts and feelings change to be like Christ. That is the real battle of being Christian. But it's the best thing you can do.
God be with you.