Bit hard to give you proper advice without you posting on the actual warrior you’re playing or some logs of how you press buttons, but with a little sleuthing and it being your ‘main’ when checking you on raiderio.
I’d start with the following from checking out your talents and logged keys.
Play more in defensive stance. - This one might be silly but you don’t really have any stance changing if at all during a key and one that you do (RLP) you’re only in a stance for ~68% of the key.
As prot you need to know when you’re able to dish out more damage in battle stance and when you need to hunker down in defensive stance to mitigate the incoming damage.
As a starter, maybe sit Defensive Stance on Trash packs and go battle stance for boss fights, while switching to defensive stance for tankbusters. This will give you more comfort in swapping your stance as needed.
Maybe set-up a WA that covers your screen with the wording of your choice, maybe something ‘mean’ to tell you if you don’t have a stance active. This is a pretty common occurrence if you’re always changing between specs as your stance likes to be removed when doing so.
Talents, begin with making the following changes.
Tough As Nails to Spell Block. (VERY good magic damage mitigation)
Impenetrable Wall to Defender’s Aegis. (two charges alongside a shorter cooldown to feed into with AM is pretty solid and allows for much better planning of defensives to survive bosses with ease on tyrannical weeks and even larger mob pulls on fort weeks)
Optional* Fueled by Violence to Brutal Vitality. (I say this one is optional as they’re both a little chop/change but i’d say BV has more value when you’re going ham in battlestance on a large pack as you’re able to benefit a fair bit from the aoe damage you’re dealing during burst).
Why? This will give you more magic defense which is something you’re admitting to struggling with.
Finally get back to basics.
Press your Shield Block on cooldown, your uptime is lacking really painfully. I cannot stress this enough Press Shield block.
You have a much larger % of unblocked melee attacks than blocked. This is a really bad thing as your block is your main mitigation tool with large packs of mobs, seeing this i’m doubting magic damage is the only issue you’re having.
Press your defensive cooldowns on cooldown.
You could be mitigating more damage by actually using defensives more often. You don’t need to hammer them all into a one shot macro, but you could do with staggering their use as a pack progresses. Opening with your Shield Charge into Demo shout, wait 10s then press shield wall once mobs start doing their tank busters, bleeds etc. All while spamming the living end out of your shield block/ignore pain.
TLDR:
Go back to the basics.
Make sure you’re using your defensives as often as possible.
Make sure to keep shield block up.