Truth

In happiness context, it simply means, law that if it’s practiced rightly, brings happiness
its contrary is untruth, it simply means, one that brings unhappiness

Truthful

In happiness context, it simply means, one that practice the truth
its contrary is untruthful, it simply means, one that practice the untruth

Truthfulness

In happiness context, it simply means, the degree of being truthful
its contrary is untruthfulness, it simply means, the degree of being untruthful

Hence,

Truthful = Happy

Hence,

Truthfulness = Happiness
Happiness is directly proportional to truthfulness
The more one is truthful the more happiness one will get