Also, make absolutely certain you're moving left and right according to the way the mouse is being held. A lot of people have the tendency to hold the mouse slightly angled, and this can mess up your blocks. I had to relearn how to hold a mouse to play this game.
This.
Go to duels server more often , play defensive and try to observe how you react to enemies attacks.By reacting i mean where do you move your mouse =)
Your problem is probably this.
You have 2 options to solve this
Either you will learn how to hold the mouse properly or you will get used to move your mouse to any direction according to mouse's current position.
It is a hard and painful process i am playing this game for 1 year yet i still suffer from it
edit:I even tried using keyboard for blocking but it is reallly hard to adopt that.
I know one guy who uses keyboard to define directions of attacks/blocks and that guy is Cicero , he started first Mount&Blade singleplayer with this setting and from that time he's been playing with it that's why and how he's good at it