Forgive me if I get a bit technical in what follows, if it is not helpful please disregard.
A mortal sin, like the word quite apparently means, is a deadly sin-- one which is capable of removing a person from grace.
A venial sin, which just means “pardonable,” has something of the nature of a fault, which while it does not bring us to God (it is a sin, after all) we do not remove ourselves from God’s grace by committing it.
Augustine says there are two sorts of people, those who live for God and those who live for themselves; if you love yourself to the exclusion of God-- more than you love God-- then you sin mortally.
We talk about sins which are of “grave” matter, which means that by their nature they remove one from God’s grace. One cannot break the Ten Commandments with malice and not sin morally. Once I kill a man or fornicated, no matter my motive (leaving non-culpable ignorance aside), I have sinned to such an extant that my soul is, in a sense, dead. Because of God’s infinite mercy, I can still be forgiven in the Sacrament of Penance, but I have cut myself off from his graces.
Some sins are in themselves venial (pardonnable) but can be directed in such a way as to remove me from God. If I tell a harmless lie, but I do it precisely because I love that lie and myself to the exclusion of my love for God, I commit a mortal sin.
OK. So, now, lustful actions are of two kinds. First, we consider (to a certain extent) the emotional or biological response to someone of the opposite sex to be a lustful “movement of the flesh,” for instance. While this can be a prompt towards sin, and is at least sometimes a venial sin, it is not a mortal sin.
Second, we consider lustful intentions; here we have another distinction. First, the intention of the sin makes the sin to be what it is. Let us say that I want my boss dead and I try to poison his drink but it spills. Am I a murderer? Although I am not successful in the external act, I am still a murderer in my internal act or intention. So, also, when I intend to fornicate with an attractive man, even if I don’t actually go through with it, I am probably mortally sinning; because, again, in fornicating one removes himself from God’s grace.
Anger can be considered in the same way, but also with the caveat that there is bad anger, and righteous indignation, as the last post nicely said.
Remember always the infinite mercy of our God.
Jessi