This varies a lot depending on if you're playing on console or PC, but there are a few tricks you can use to help yourself see through the clutter.
The first thing you will probably want to do (especially as Pld) is slap a marker on your healer ( "circle," "triangle," 'target to ignore," etc). You can pick whatever marker makes it easier to see them in the crowd. That way you always know where they are, can keep them in range, and maintain line of sight. In emergencies, it also makes it easier to throw up a Cover or Clemency cast.
The second thing you'll want to do is cycle through all of your opponents. You want to do this quickly at the start of the match (as soon as they're in range) so you can get a feel for the field. Things to look for are, 1) who has the offensive buff? 2) who has the defensive buff? and 3) where is the healer?* Knowing which targets have the defensive/offensive buff will give you an idea of which targets will be attacked by your dps team, and which will be ignored. You can also anticipate what's going to happen, and which target is best to use Full Swing on (though, ultimately, the dps will be the biggest deciding factor). It also lets you know which target is the highest threat to your healer, meaning that you can keep an eye on them and hit them with the occasional stun to interrupt their bust. *Most importantly, it allows you to identify the enemy healer before combat has started, and Focus Target them.
I Focus Target the enemy healer for two reasons. The fist, is that I can always see their cast bar. I know what they're casting and when they're casting it at all times. Secondly, Focus Target marks the healer with an arrow. It's not as noticeable as a marker, but it's better to use because it avoids confusion. I don't want to put a giant circle over the enemy healer's head and accidentally make the dps think I want them to burn him/her. Focus target allows me to keep up a visible marker without imposing it on the rest of the team.
After that, actual targeting comes down to practice. Tabbing through targets is not nearly as fast as point and click, so it's best to get comfortable with adjusting your camera on the fly to give you the best possible lines of sight on your enemies. An elevated view tends to help with this. On console it's a little trickier, since we don't have a point and click option. I tend to use what I call the 'circle jerk' method, which relies on good auto targeting. That method also relies on keeping the camera in the best possible angle. A direct line of sight will often land you on the target you want.
I wouldn't worry too much about your Dps when you're playing on Pld. On War, high output is your primary utility, so it's all about the Dps. On Pld, though, your time is better spent exercising your maximum CC and utility. My best matches on Pally have seen Dps numbers as low as 40k to as high as 90k. I average around 50k a match, which is pitiful, but there's a damn good reason for it. Most of my time is spent identifying pesky targets and stun-locking them. Unfortunately, outside of a few oGCD's, Pld's cannot Dps and Stun lock at the same time. This means that, during a full duration stun lock (9 seconds), I'm outputting almost no Dps. What I am doing, though, is buying the rest of the team time. If I can stun lock a Healer for 9 seconds, then my Dps should (and they damn well better) be able to murder something in that time. If they didn't, then the fault is with them, not me. You are right that you have to divide your attention a LOT as a Pld, though. Playing it well comes to making good judgment calls base don the circumstances, so there's not universal guidebook. There are, however, a few tips and ticks to use to your advantage:
The first is your CC timer. After 3 stuns, the target resists. That resist lasts a full minute, and a Pld's Dps is not high enough (even at maximum burst) to cause interrupts to cast bars without it. Meaning that, while a target is resisting, you're just wasting your time if you keep smacking them around without additional Dps support. Do something else. Check your party. If the healer needs help, go help. If not, help the Dps. After a full minute has gone by, you can go back to stun-locking.
Secondly, spread out your stuns. Pld's have the most fluidly applied stun in the game, so you can hit a lot of targets in a short time. Don't be afraid to smack someone with one. Your primary targets are whoever has the offensive buff, or anyone using their burst. You have to look for the tells. For example, see a War using a Fell Cleave? they might be starting their burst of a double or triple Fell Cleave. If you really want to piss in their cornflakes, smack them with a stun and watch their burst fall apart.
Another thing to note about stuns is that you do not have to use their full duration. Even a single stun can really throw a wrench into things if it's timed well, because stuns are primarily used to buy your healer time to keep up with the damage. The only targets that you should always be using a full stun-lock on is the enemy healer, because it maximizes your dps team's up time. Otherwise, a single stun is often enough, and it allows you to stay mobile and active. You shouldn't be stunning all the time. The CC resist timer means that you can waste a stun. So use them when they're actually useful, and remember to apply your DoT's in the meantime.
Party defense is less tricky, because it's used as needed. You should not be disengaging to re-apply buffs unless you absolutely have to. In the heat of combat, it's better to use moves like Clemency, Divine Veil, Cover, or Testudo to protect the team. Stoneskin and protect are best left unused unless there is a lull in the combat (such as if the enemy healer dies and their team retreats to regroup). You should also be prioritizing who you use those buffs on. Targets that have Heavy Medal vulnerability take priority, especially if you're low on mana. In general, Testudo and DV are best used in an alternating fashion, because they have a lengthy CD. However, if the teams are evenly matched you may want to hold off on using them. If you expect a "Culling Time" to happen, you'll be happy to have those party buffs at the ready. Otherwise, use them as needed. Cover is trickier, because you have to be very quick on your application, and it's not always easy to tell when it's actually needed. Most often, it's a roll of the dice. However, in the case of healer's it's always a good idea to use it on them if you see them activate "Attunement." That's a last ditch effort for a healer, so if you see them locked in place and dodging a shit ton of attacks, throw up a Cover and a Clemency to help them out. It will be especially useful if they don't have any oGCD heals at the ready.
As for Full Swing, in the case of Pld's it's more of a party support ability than a personal Dps boost. War's can see an immediate effect, but they have a very strong burst. Pld's do not, so it's best to use Full Swing on whatever target your dps are trying to burn and then swapping to the healer to suppress their heals as much as you can during that time. For this reason, I typically time my Full Swings to the stun resist of the enemy healer, so that I can use them in tandem. I spend most of my time on Pld supporting the Dps. I only, grudgingly, go back to healer defense if they look like they're having a hard time.
Glory Slash is often better used for extending your healer suppression. The full Pld lock-down is 3 stuns (timed for full duration) followed by a Spirits Within (silence) and a Glory Slash, because the knockback interrupts a cast. That said, early in the match you may want to vary it's utility. The reason for this is that, after 3 stuns and SW, the enemy healer will be utterly desperate to get a heal off. Most enemy healers will panic and burn a oGCD like benediction, and you can't interrupt that. The good news is that you forced them to burn it early. The bad news is that they probably saved the target and neither SW or GS will have prevented it, so it might be best to go for maximum damage output instead of maximum lockdown. That's a roll of the dice, though, since you can never truly know what the enemy healer will do or how fast your dps are. If they kill the target before your stuns even wear off, then you might as well use GS for it's utility rather than it's interrupt. It's a case by case basis thing, and you have to be very aware of what's going on around you to know the best use.