You should always open with 2-3 wounds depending on speed. Then switch to wound + block to minimize damage to your HP using the small shield. Do not open with stun, instead start stunning after you begin to block. You can get 2 wounds in when stunning.
The result is that your HP and shield HP will always be at 100% roughly and you have minimum downtime. Like others have said, you can always deplete your shield completely since it will instantly regen after some downtime. The thing about this is that it is slightly slower in terms of kills per minute.
You should always be working two stuns into this rotation as well. So essentially you get a 6+ wounds almost for free before you factoring in the number of blocks you can get.
You are fightiing mobs too higher level if your shield is depleting too fast. Your goal is to fight red enemies 1 level higher, not 2 levels higher. Avoid poison enemies.
Playing warrior is exhausting, especially after some recent changes though today’s patch made it a little better again.
Does it get better on higher levels? Not really if you know what you are doing. With the small shield you should be killing each mob with very little HP loss and you’re ready to pull the next mob with only a few seconds of downtime.