first, stop reading YA fiction.
second, your characters are probably repetitive because it's a type of character you enjoy and are comfortable roleplaying. aka, everything else feels foreign and you come up blank.
now, the real solution. look at what you roleplay and do the opposite. instead of looking at things from YOUR point of view, look at them from the character's. it may also help to play a different age group or gender. most importantly, stick to your guns. if your character's shy as fuck, they better stay that way till something occurs that can help them reach past that.
finally, so what if your characters are repetitive? you're not writing a fucking book nor are you some damn english doctorate. what's most important is that you ENJOY writing for the character. because, let's face it, you can play someone completely different, hate it, and get fucking nowhere. test the waters and see what you get out of it.
and, if all else fails, stick to what you know.