Dear John:

Grave matter is never relative, it is objective. Even if circumstances cause a "diminished culpability" whereby the grave sin is not counted as mortal to us, it is still objectively grave.

For example, masturbation is a grave sin. As mentioned in the Catechism "affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors [may] lessen, if not even reduce to a minimum, moral culpability"  but it does not change the objective reality that it is grave matter.

God Bless,
Bro. Ignatius Mary