Is X a Mortal Sin?

It is common to wonder if a given act, whether committed or not by anyone in particular, is a "mortal sin". The importance of this question is grave:

Baltimore Catechism #3:

Q. 280. What is mortal sin?
A. Mortal sin is a grievous offense against the law of God.

Q. 281. Why is this sin called mortal?
A. This sin is called mortal because it deprives us of spiritual life, which is sanctifying grace, and brings everlasting death and damnation on the soul.

So, asking if a given act is a mortal sin is very important, but it is easily forgetful of the definition of a mortal sin:

Q. 282. How many things are necessary to make a sin mortal?
A. To make a sin mortal, three things are necessary: 1.a grievous matter, sufficient reflection, and full consent of the will.

A given act therefore can only be a grievous matter. Many things can be grievous matters. A mortal sin can be committed with a thought as well as an action. It can be known by others, or others may never be affected by it.

QQ. 284. What does "sufficient reflection and full consent of the will" mean?
A. "Sufficient reflection" means that we must know the thought, word or deed to be sinful at the time we are guilty of it; and "full consent of the will" means that we must fully and willfully yield to it.

This is a tricky thing to answer for another, and sometimes for oneself. Therefore, the best and surest thing to do is confess the sin and give all necessary facts to the priest. A trusted spiritual director should also be obtained if possible.

If we will it, we can easily deceive ourselves. So, if we wanted, we could convince ourselves that the will or knowledge was deficient to commit a mortal sin. Now, individually, dealing with ourselves varies. Some people may be prone to deceiving themselves they have not committed a mortal sin, and others may fear that all imperfections were mortal sins. It is not my place to even pretend to be able to answer such questions specifically. I barely answer themselves for myself even though I do not have major problems with any spiritual reflection on my acts (for which I am grateful).

A general rule I find is that is one is actually distressed at one's sins to any degree, that one probably is suffering from a bad habit, scruples, or another distress and it is not a case of a purposeful commission of an a known evil act. If one is looking for justification for an evil act, seeking to be comforted for acts which one chooses freely to commit, or trying to find an excuse, then one should change the behaviour.

But whatever the case, following the instructions of an authority in spiritual matters should resolve most conflicts within oneself.

Living in this world is not joyful, and often sorrowful. We may be tormented by bad habits, social or personal rejection, mental, physical, and spiritual weaknesses, or even harm, but our bodies are mortal and the soul is immortal. God is all knowing, all powerful, merciful, and just.

We are told to strive, to persevere, to constantly pray, not to find a level and stay there. A constant improvement, a constant striving, and a constant grace is necessary. It is not what our flesh desires, but it leads to Eternal Salvation.