Tanking via base class most mitigation comes from simply having up to date gear and while this always holds true throughout a tank's career it's noticeable in the early levels because they have none of their damage mitigation tools. Although GLD does have 1 damage reduction action (I'm talking like early game dungeons). When I leveled CNJ (and later WHM) I never noticed too much of a discrepancy between having to keep either of them alive.

It doesn't sound like its your fault based on what you said but then again:

"It's just that you're a healer in a MMO which means you don't matter...until you do and by then its completely your fault your group lost."

Anyways j/k on that last bit I think you'll be fine although all the above advice is very sound advice and you'd do well to take it. Best of luck on your healing career and don't let it get you down when you have a bad run. We all have them.