The Catechism of the Catholic Church contains the deposit of faith. We must accept it and assent to it.
Paragraph 1735 says that not all grave actions are mortal sins. 1735 is, depending how you look at it, either very vague or very general. But an individual action is not necessarily a mortal sin, even if it is intentional.
Each and every person is called to be holy. In itself a temptation is not a sin. Temptations tell us where we need to improve ourselves to be holy. Even if a person is young, they ought to develop a way of working past a temptation. The mind must focus on something else such as prayer or meditation or an alternate physical activity to distract us from sin. Even reading an ordinary book can help us past a temptation.
In the Catholic Byzantine Rite, the Hail Mary was not a popular prayer. Instead, people would and still do say the “sinner’s prayer” Lord, have mercy on me a sinner.
To some or even many, this is foolishness. But, we don’t care about that because we are disciples of Christ.