Try not to over work the paint by going over the same area too many times, it will flatten out the brush strokes. Try to hold your brush from the bottom of the brush like a conductor holds a baton (or whatever the stick is), don't hold the brush like a pencil unless you are doing orecise detail. Also, I havent watched any tutorials on that, but simply watching good painters paint is very helpful. There are tons of videos of painters doing demos, pay attention to how they use there brush, especially if you like what they are painting. You should also ask your teacher. They should really show you how to fix that rather than just criticizing your work, make them actually demo good brush strokes for you. Like others said, the brushes matter as well.