I had the same feeling when tanking on my JK Guardian. Was frustrating like u say that after using almost every ability u had in the book mobbs would stil peel of u. Also the makeup, size and how they are sometimes spreadout really make it difficult
But then i changed my way about how i approach mobbs in flashpoints.
Now i just tell my dps to kill the weaker targets in a trashmob first while i work on the stronger ones (sliver and gold elites)
Lets say the mobb is made up out of anywhere between 2-5 weak targets, 2 silver- and 1 gold elite.
Like i said, the weaker targets are not my priority at first. I saber toss one silver elite, jump to the other and forcesweep the gold elite, and keep working up aggro on those targets, while in the mean time the dps can singeltarget or aoe the weaker ones. Only time i intervene is when i see a dps struggling with his target or if one of those targets is gunning for the healer. This has really made it more easier for me on how to handle mobbs.
Also knowing the flashpoints helps. Sometimes when engaging a mob, more adds will come in, either through a side door or they drop in from above, those are times when i use my aoe taunt just to grab them all before they start running loose.
On a single target i agree that our aggro generation could use a boost. I usually run with a sentinal dps who is about equally geared as i am and im struggling just keep the target from attacking him. When he goes all out and uses cd,s or relics i know the target will peel right off me. At that point my i just keep a taunt at the ready just to get aggro back.