If you're not already sick of all our advice, I got one more thing to add (which I hope hasn't been mentioned, with all the walls of text going on
). I've seen you mention that in certain boss fights, you feel you're pretty much done for when your companion falls. Since you're a Tank, though, the boss should always be focusing on you, not your companion. There are two useful skills for this, Taunt (which taunts one mob at one time) and Challenging Call, taunting all mobs within a certain space. So what I'm trying to say, in the not-so-ideal case you're no match for a boss, it still should be you who dies first, because you have his attention.
That's what you should keep in mind. You're the tough one, your companion does the most damage (or heals). In all honesty, I haven't found the Jedi Guardian spec'd Tank as that much more a challenge than other classes. I tend to breeze through groups of mobs, almost as fast as my merc does. But yes, a Guardian spec'd DPS should be easier to play. So if it really doesn't work out, respec!
In any case, I've got the same class and role as you, so if you need further advice, you may always ask.