I have a 31 defiler and I experienced some of the pain you are going through right now. However I think I have a few tips that might help.
First, yes you should try and upgrade the type of drink you are using but it is not absolutely necessary. Second, continue to Ward before the fight. Only debuff is you are fighting tough single mobs, or really tough grouped mobs. By single and grouped I mean is there 1 of them or 3 not just their con. If you are fighting a 3 mob pull of blue cons with no arrows do not debuff. However if you are fighting a 2 mob pull, yellow con, with 1 arrow or 2 arrows each deffinately debuff both of them. Expect to burn power as the pulls get harder.
Now, a healing technique. Let ward break and do not recast immediately. Allow your tanks HPs to turn yellow and then cast your largest heal. If I'm not mistaken it should be the half sec cast time one. This should place him pretty close to full HP. Then recast your ward. If your tank is the only 1 getting hit this should give you approx 10 seconds of you standing there with nothing to do. The better your drink the more power you can recover in that time. Also, ask other group members to help out using any stuns and debuffs they may have.
Stay away from using DoT's and lower level heals if at all possible because these just eat power and aren't worth the effect. Pretty much your spell casting should be ward (for pull), debuff, heal, ward, pause, heal, ward and maybe 1 last heal.
The final tip. If you are the only healer in a group then you should have a lot of damage output or at least an enchanter with you. If this is true then yellow or orange single mob ^^ pulls should die in about 20-25 seconds, maybe 30 tops. You shouldn't have to cast more than 3 wards and 2 heals total in that fight. If you are, then maybe the challenge your group is taking on is to great for the abilities you all have. Not just you as a healer, but your tank and damage people as well. I would suggest finding the next step down from your current target and fight those. It will be much more efficient, safe and rewarding. Also, a lot less frustrating.