First off, you may want to look on the tanking forums. There's an excellent post which looks at ideal stat distributions for various mitigation budgets. I'm pretty sure your defense is too high, and your absorb is too low. It would depend on what relics you're using though.
Second, and more importantly, your log shows that while you are *taking* slightly less damage than other geared assassin/shadow tanks (by about 15 DtPS relative to my logs), you are requiring significantly more healing. You don't appear to be using your self heal…at all. For example, on Kephess (a fairly dynamic fight), you had 82.4 HPS on your self-heal. When I did TFB HM yesterday, I had 132 HPS on Kephess (http://www.torparse.com/a/67258/29
), which is really on the low end for me due to the movement and burst phases. On Writhing Horror (http://www.torparse.com/a/67258/7
) I had 178 HPS, and actually over 200 once you adjust for the the silly 15 second delay before the fight ends after the boss is dead. On that same fight, you had 68 HPS *and* you took more than 100 DtPS more. That means you required more than 200 HPS more out of your healers.
That's fixable though. It really has nothing to do with your build and more to do with your rotation. Try using Force Lightning only on the Harnessed Darkness proc. You appear to be using it primarily without the proc, which is sort of pointless since it doesn't really do that much damage until you have 3 stacks. You're also using Maul (which is a HORRIFYING force dump for very little damage without the proc) and a not-insignificant amount of Thrash. In fact, your primary damage output seems to be coming from using Force Lightning on cooldown regardless of proc, which is why your self-heal is so low.
In any case, I agree that mitigation is king, but you definitely have some improvements to make if you want to maximize your *survivability* (external healing required).