Dear Questioner —

Mortal sin requires three things:

1. Objectively grave matter (it's serious)
2. Knowledge (you have to know it's sinful and gravely so)
3. Full consent of the will.

If you are struggling with a sin, you may fail to meet #3. Factors such as habit, addictions, coercion (either external or internal), passion, tiredness, stress, impulsiveness, and so forth can mitigate the full consent of the will.

So, for example, if you are half-asleep and do something, that's not full consent of the will.

Or if you find yourself stirred with passion, despite taking precautions and haven't initiated them wilfully, and you act on those passions, that would not qualify as full consent of the will.

I urge you to go to Confession and discuss specifics with the priest.

He can help you discern things.

Eric