Yes, it's simply not doable for me. I've heard of a few people being able to do it, but I suspect they're burst specced Sentinels or some such thing, capable of burning things fast while using defensive cooldowns.
I am tank spec as well, and it's simply not possible without serious upgrades. I'm not even sure it's possible then.
I assure you, even with all my cooldowns popped in pretty good gear my DPS as a Sentinel wasn't even close to being enough. I had to get help, as has every other JK I know.