Just a quick suggestion, but have you levelled up the shadow spells from your trainer? Some, like MB and SW:P will have been bought while you were disc, but Mind Flay and VT may be lower levels and just tickling the mobs instead of hurting them.
Not that I've ever done anything like that before.
As far as rotation while grinding, I would guess that you are using VT/MB combos to keep the replenishment going, adding VE in before the MB if you are taking damage already, after MB if you expect to take damage soon, and adding DP and/or SW:P before Mind Flays if the situation requires it, such as tab dotting extra mobs before returning to your primary target.
If you are simply being overrun by the mobs, all I can suggest is to slow down a little and plan out your pulls from max range. Shadow can dish out some damage, it just takes some time to ramp it up. Use all your tools such as shield, fear, dispersion... heck, MC a mob and make them beat each other for a bit for some chuckles. (You lose your opening when you do this, so have a plan in mind when your MC breaks and you have a couple angry mobs coming at you. I usually would fear, dot, and then start up on the healthiest one with the VT/MB combo.)
Just one question on your spec. You have Shadow Weaving filled out before Darkness. I don't think there's much difference in the numbers at that gear range, I am just curious whether the 3 points in Shadow Weaving for the stacking buff to 10% would be better used in Darkness as a flat 6% damage gain.
I switched off of shadow around 65 and didn't pick it up again until 80, so I may have missed something pertinent to that level. The one thing I can say is that you cannot rely on your shield and heals like disc can. I found that I had to resist my temptation to fall out of shadowform to heal myself. Once you hit Holy Nova, you may as well finish the fight as a healer would instead of relying on the strengths that shadow provides.