It’s been a while, so let’s refocus on just the basics first.
As a healpriest, these are Your Rules. Rule the first, The Tank Shall Stay Alive. Rule the second, You shall stay alive except when it violates Rule the First.
All else is gravy. And details.
Shadowpriest, this is your rule. Thou shalt maximize thy utility.
This needs expansion, therefore… You will, given equal gear and skill, always have other classes that can do more DPs than you. The only reason for bring you instead of them to the party, then, is what you can do which they cannot. You have three things and only three to bring – know this and be prepared.
First, once you are high enough level you can provide mana regeneration. If you are high enough level and do not have this, then the remaining two elements are unlikely to be sufficient to bring you to the party.
Second, you bring a shackle. So do healpriests. And hunters can trap the mobs you shackle. If all you bring is a shackle it is often not enough, and you should not be surprised if you are not invited to many parties.
Third, you are an emergency healer. It is comforting to have such even if never called upon. However, emergency healers also abide in Shamans, Druids and Paladins. You are not unique, and it becomes a great temptation to leave that to the other classes. Know, however, that if you decline to prepare for and provide this you should not be surprised if you are not invited to the party.
First and always, maximize your utility. Within this lay several details, a critical one being max dps without crippling your utility. If you sacrifice utility for more dps, there are others who can – and WILL – gleefully take your place.