You seem pretty talented! I am also no expert, but as others have said you should be sitting higher and farther away from the keyboard. Your forearm should be coming in basically horizontally, and your wrist should be level. Right now your elbow is slightly lower than keyboard level, which means that your forearm is coming up at an angle, which means that your wrists are bent, which tires them out in the long run. Get a teacher and ask him/her to show you!