A little indecision would have been a good thing in Turin's case. Decisive action, when they are inevitably wrong actions, is no virtue.
Turin's problem is he never stopped to *think*, for example to wonder how Glaurung could know about Morwen and Nienor's plight in the first place - much less to doubt the dragon's motives for telling him this. Turin's efforts to avoid his curse were limited to changing his name, not his attitude or his behavior.
Manthor makes Hurin calm down and *think* and begin to act rationally again. unfortunately Hardang ruins the effort by drugging Hurin so Manthor can't continue the treatment. Melian too simply talked to him reasonably and sympathetically, helping him to calm down and see things straight. I very much doubt she used any kind of power, and even if she did it would have been as useless on Hurin as it was on his son *unless* he were still open to the truth. Melian didn't 'heal' Hurin, she helped him heal himself.
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