If the fight has no party wide aoe, and dps are dead. It's their fault. Your main priority should be keeping yourself and tank alive.

If you are wondering how good you are as healer, esp whm, you can try considering this.

1) How soon you can fill up HP lost for your tank/party.
2) How high is your MP wasted on overhealing.
3) How fast can you remove bad status effect.

If you are bore, and want to polish your skill more outside CoB, you can solo heal 8 man content, like Garuda HM, Titan HM.

Lastly, if you stuck to some point that tank keep dying, you can try analyzing combat log to learn from mistakes.