I read on some sites that strength is great because of how much parry one gets from strength. When I was gemmed and reforged avoidance I stayed alive a lot longer but had issues with threat. I decided to gem and reforge more control (7.5% hit and 15% expertise). Now it seems like I take so much damage the healers have a hard time keeping me alive even though I am popping shield barrier on fights like the council 10 man regular and shield block when tanking horridon. I use thunderclap, demoralizing shout, my banner and I still die all the time... This is getting very annoying and I am holding the raid back. I am about to stop raiding because for weeks after spending thousands of gold reforging I have not gotten much better.
Have you got any logs of your fights? it's sounds like a playstyle, either getting caught in AoEs or just not managing your rage or active mitigations properly. However You should be prioritizing Mastery over avoidance, and perhaps over Stamina as well depending on the content you're doing. Ideally you really don't want to be gemming strength or avoidance, but mainly stamina and/or mastery, and using the Eternal Metagem.
Have a read of the guide in my sig, its pretty informative.
We'd definitely need to see some logs before we can help, but it is also possible that it isn't you that's the problem.
ToT is meant to be a gear check of sorts, and if your healers and DPS are a bit behind (LFR on Sundays and Mondays, doubly so -- Sunday I was in a Part II run where 10 of the DPS were below 30K, and two of them were just autoattacking) it may look like you're the squishy one, but your gear is right about where it should be for ToT. On fights like Horridon in particular, lower DPS means more adds with mechanics that just eat a tank alive -- I run with two groups currently, and the group with the higher DPS has an easier time of the doors than the group with the lower DPS, where we get overrun sometimes on the first door.
Your tanking movement style is another factor -- when you're tanking the cloud of adds on the door, do you gather them while keeping your front to the mobs, or do you run about and grab everything regardless of positioning? I know when I'm tanking them it's really tough to keep everything in my front arc, since they come from all four directions, so I wind up slewing to the sides to minimize the amount of time I leave my furry butt facing the larger amount of mobs. If you're running directly towards new adds on this fight and presenting your back to the existing pack that hasn't been killed yet, you're negating all of your dodge and parry ability. So of course they're going to stab you in the back. A lot. And ToT mobs hit very, very, hard compared to their MgV or HoF counterparts.
Teng is by far and away your expert here on warriors though.