I'm also guessing you're mostly having trouble with DK and Warrior?
Both of these classes counter Spriest pretty hard. Void Tendrils is a good choice to allow you to get a couple of casts off (Get your DoTs ticking and what not, which allows the use of more instants). Also make use of Phantasm. It's great for making distance when they're on you or getting a cast off from range before you get gripped/charged. You won't 1v1 them, anyway. I doubt you'd even 2v2 if both classes had a pocket healer. In 3v3 you stand more of a chance with someone helping you on damage and someone healing you through them training you. A healer gives you more time to fake / set up damage, I never get mad about 1v1 or even 2v2 situations.