I hope this will help. It is from the older Baltimore Catechism.
Q. 282. How many things are necessary to make a sin mortal?
A. To make a sin mortal, three things are necessary: a grievous matter, sufficient reflection, and full consent of the will.
Q. 283. What do we mean by “grievous matter” with regard to sin?
A. By “grievous matter” with regard to sin we mean that the thought, word or deed by which mortal sin is committed must be either very bad in itself or severely prohibited, and therefore sufficient to make a mortal sin if we deliberately yield to it.
Q. 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.
Q. 285. What are sins committed without reflection or consent called?
A. Sins committed without reflection or consent are called material sins; that is, they would be formal or real sins if we knew their sinfulness at the time we committed them. Thus to eat flesh meat on a day of abstinence without knowing it to be a day of abstinence or without thinking of the prohibition, would be a material sin.
Q. 286. Do past material sins become real sins as soon as we discover their sinfulness?
A. Past material sins do not become real sins as soon as we discover their sinfulness, unless we again repeat them with full knowledge and consent.
Q. 287. How can we know what sins are considered mortal?
A. We can know what sins are considered mortal from Holy Scripture; from the teaching of the Church, and from the writings of the Fathers and Doctors of the Church.
Q. 288. Why is it wrong to judge others guilty of sin?
A. It is wrong to judge others guilty of sin because we cannot know for certain that their sinful act was committed with sufficient reflection and full consent of the will.
Q. 289. What sin does he commit who without sufficient reason believes another guilty of sin?
A. He who without sufficient reason believes another guilty of sin commits a sin of rash judgment.
Q. 290. What is venial sin?
A. Venial sin is a slight offense against the law of God in matters of less importance, or in matters of great importance it is an offense committed without sufficient reflection or full consent of the will.
Q. 291. Can we always distinguish venial from mortal sin?
A. We cannot always distinguish venial from mortal sin, and in such cases we must leave the decision to our confessor.
Q. 292. Can slight offenses ever become mortal sins?
A. Slight offenses can become mortal sins if we commit them through defiant contempt for God or His law; and also when they are followed by very evil consequences, which we foresee in committing them.
Q. 293. Which are the effects of venial sin?
A. The effects of venial sin are the lessening of the love of God in our heart, the making us less worthy of His help, and the weakening of the power to resist mortal sin.
Q. 294. How can we know a thought, word or deed to be sinful?
A. We can know a thought, word or deed to be sinful if it, or the neglect of it, is forbidden by any law of God or of His Church, or if it is opposed to any supernatural virtue.
Q. 295. Which are the chief sources of sin?
A. The chief sources of sin are seven: Pride, Covetousness, Lust, Anger, Gluttony, Envy, and Sloth, and they are commonly called capital sins.
Baltimore Catechism #3
I think this points things out fairly clearly.
Note that in order for the confessor to accurately judge whether your sin is mortal or venial, you must give him enough information about the nature of it, your motives, and other factors.