make sure you are blocking your enemy’s attack from the right direction, or the frogs will make quick work of you. All weapons can block in all four directions. Hold the right mouse button while moving the mouse in the direction you wish to block. To release your attack, let go of the left mouse button. Swords, for example, have four attack directions: up, down, left and right. Your bayoneted musket only has two direction: up and down. To attack, press the left mouse button while moving the mouse in the direction you wish to attack. To do so, either press Z again or simply use any of the movement keys. This is very effective against frontal cavalry attacks.
Since you are also in melee mode you can see that your character is bracing his bayonet.