Welcome to the world of PvP Tanking. You've pretty much hit the nail on the head. Tanks have FAR more things to split their attention across than any other job in the game, and it's not easy to manage everything. In my experience, the key to being a "good tank" in PvP comes with how well you can make judgment calls on where you are best needed. That varies from match to match, circumstance to circumstance, so there's no universal rule book to follow here. There are some pointers we can give you, though.
First off, a disclaimer: I'm also, by no means, an expert. I derp around just as much as the next guy, so if anyone sees things they don't like here, feel free to correct me or offer your two cents on it. I also spend most of my time on Pld, not Drk, so my advice may need to be adjusted accordingly.
The first thing you're gonna want to do in every match is establish how much or how little healer support you're going to need. You won't know for sure until the match starts, but you can make educated guesses based on the job. Here's the general rules of thumb that I follow at the start of each match; though, keep in mind that things can change dramatically depending on the skill of your healer.
- Astro's: They can paper tank pretty well, so minimal support if they're good at their job. They also tend to have a harder time healing multiple targets at once if the damage is being split, so that's all the more reason to spend more time supporting your Dps. Stun targets accordingly to try and spare the healer from having to jump from target to target. The more crowd control you give, the stronger your astro can shine.
- Whm's: They are moderatey good at paper tanking, but much better at crowd control. If they are good at using their CC, then moderate support is needed. Basically, keep an eye on them and adjust your tactics as necessary, but you shouldn't have to baby sit them if they're any good. Just make sure to help them out if they are getting focused by multiple dps. Keeping them on focus target might help.
- Sch: A bit trickier. They tend to need high support from the tank. The changes to interruptions and aetherflow has changed this a little, but I still find myself bailing out sch's more than Whm's and Astro's. That said, I don't see too many sch's by comparison, so it's hard to say for sure.
The next thing you'll want to do is find out what the enemy team composition is, so you know where best to put yourself. The same list above applies to the enemy healer, but you also have to be aware of what kind of Tank they have. Here's another list:
- War's: Tend to be best at healer harassment and Dps support. Their slashing Debuff is very effective, and their burst provides far more potential to interrupt casts on a single target. In particular, watch out for Nin + War combo's, as they can be deadly, especially on your healer. You may have to run FAR more healer support if you see a War focusing them the entire match (again, depending on how skilled your healer is).
- Pld's: Easily has the most utility, but can also the hardest tank to play well because they have to divide their attention more than anyone else. Honestly, there's not a lot you can do to combat a Pld kit, as most of their moves are passive; however, the more a Pld has to divide their attention the harder their job becomes. The best you can do is watch them to see where they are and be somewhere else so to force them to split their attention. Pld's excel at stun chains, so if they're focusing your healer you're going to want to do what you can to get them away from him/her.
- Drk's: Hard to judge, at the moment, because they just recently got adjusted. Before the last patch I never really paid attention to Drk's, because they just didn't have anything to offer other than a bit more Dps. Now, however, they have some added utility. Strictly speaking, their kit still seems geared towards Dps support, so you want to watch out for more offensive, rather than defensive, tactics.
After that, it comes down to tactics and situational awareness.
As a Pld, I typically start the match on Dps support, because (in theory), my healer really shouldn't need me at the very start of the match. Keep in mind, however, that the beginning of the match is where the pace is established. You may have to swap your focus immediately if the enemy team is hyper aggressive. Again, you can, somewhat, anticipate the worst case scenarios if you know what kind of party composition you're up against. So look at that party list at the start of each match. Also, keep in mind that the goal is not, necessarily, to kill the target outright (if you do, that's a bonus). The goal is to cripple or maim the target for your Dps. For Pld, this means opening up with my GB combo (for my DoT placement), in which I buff my GB and CoS with Full Swing for maximum effect. After that, I tend to leave the target for the Dps. I will mark a target if necessary, but sometimes that's hard to do in combat. You have to pick your target carefully so you line up well, or mark in advance.
For Drk's, this probably works differently, as you have a stronger burst. Burning it all at the get go is a waste, especially without the dps to support you; so, you'll probably want to save the majority of your burst until you know what target your Dps are going for. Support them as necessary.
After that, it becomes a question of "What ifs:"
IF my healer is being pressured by enemy Dps, then I might want to swap to healer support. For a Drk, that probably means interrupting the Dps that is chasing them (carnal chill or tar pit). For Pld's it could be interrupts (handy shield bash), but it could also mean cover or clemency. etc.
If your healer does not need support, however, then you am free to run interference on the enemy healer. The later is the idea situation, because I want to interrupt as many heals as possible while my Full Swing is still in effect to ensure that target dies. For Pld, this means stuns and silence. For Drk, this probably means maximum burst to cause as many interrupts as possible. You're gonna want to learn it backwards and forwards because it can be a pain to do in the heat of the moment. Of course, this also depends on how self-sufficient your own healer is. So, be aware of what's going on (again, focus target can help here). Running interference on the healer is also not a one size fits all deal. They types of healers they are changes the best method.
If your healer does not need support and the enemy healer is already struggling, then it's time to put the nail in the coffin. Maximize your Dps support and clean house, then prep for the second wave.
These are just some general tips. I'm sure you'll fall into your own method the more you play, and you'll customize your style accordingly to the Drk kit. Just a heads up though, you will NEVER be able to take full advantage of all of your utility in a single match, so don't sweat it. No matter how good you are, you can only be at one place at a time. Other players tend to forget that, so if you get flak from ignorant Dps or healers, try not to take it personally.