I'm not here to bash but to provide help to becoming a better Disc priest in PvP.
Gear: I agree with most of the people here in that you should have ~65% Resilience. Until you have that, you will die fast and often regardless of how you play.
Setup: Once you have that, its time to focus, gem, enchant properly. You can see my set to get an idea. I focus mostly on RBGs with occasional Arena games.
Talents: I notice you're not using spectral guise or roots which are huge life savers. Using spectral guise before someone gets on you can save you from popping other cool downs. Furthermore, it's a 30 sec CD that drops combat (perfect for mounting or drinking). Roots are important to have because Psyfiend dies VERY easy, has a 2 second cast, and can be interrupted very easily. Our roots on the other hand can't be dispelled (lol suck it Warrior). I also run Phantasm in BGs because getting out of roots can really make a difference vs warrior and DK. Lastly, run Divine Star (atleast for now) until you can survive long enough then you can use Cascade.
Rotation: Now that we've got that out of the way, lets talk rotation. Lets use the example, You vs a Warrior. Warriors are one strong counters vs a Disc Priest. You won't always survive against one but the longer you last the longer it gives your team to help you.
Example: Running along in Twin Peaks on your mount and a warrior charges you and slows you. First thing to do is to put Power Word: Shield up, then Void Tendrils, Phantasm, then Spectral Guise and just walk away. All 3 of these abilities are only 30 seconds so you'll be able to use them again shortly.
Now you're walking away for 6 seconds feeling smug and the warrior thinks he's hot !@#$ and can kill you. So he pops Bladestorm to break his roots and sees your stealth end. He then immediately jumps on you. Your 3 abilities from before are on cooldown so now we have to actually survive. Always try to put up PW:Shield, Prayer of Mending, then fear him. Immediately after the fear, hit yourself with Penance. Assuming he's a decent warrior, he'll hit Zerker Rage within 2 seconds so at most you'll get 2 Penance ticks off. He'll charge back into you (which stuns). At this point, pop your Inner Focus, and flash heal yourself for ~90,000 with a 90,000 DA. Put up another PW: Shield and laugh maniacally as he tries to break ~180,000 in Absorbs.
I also forgot to mention, if at ANY point you get interrupted, throw out a Divine Star. It doesn't get holy locked and with a 3 stack penance it heals for ~40 each way.
If you have specific questions, etc; I'd be happy to answer them.