I think this is the biggest issue with mouse vs. controller. Lets say you're being attacked by the enemy so you're experiencing hit stun. During this phase, you can't change the directional input of your guard. On controller it's as simple as holding your joystick down in the direction you want to guard. The moment you exit hit stun, your guard will input and you will switch guards as quickly as possible.

With mouse, you can't simply 'hold' a direction. Your input is only registering when you are moving your mouse. So if you are in hit stun and want to move your guard up, you need to do it AFTER the hit stun ends or you need to time it so that your mouse is moving upwards as the stun is ending so that your guard is queued.

Does this all make sense?

The solution is to allow for queued inputs while in hit stun, or basically to not disable directional inputs while stunned.