…and note that frenchbeatle states that he/she goes to confession “every month or so,” which undoubtedly helps him/her avoid mortal sin.
One thing I want to say, though, speaking not of any particular person on this thread – making no assumptions.
(1) As we grow in the spiritual life, we begin to see both our venial sins and our sins of omission as far more serious than we once thought. The saints & mystics describe this phenomenon as being repelled, horrified, by the tiniest divergence from virtue. Recall how, for example, Faustyna became acutely aware of how significant each omission of kindness could be – how when we want passionately not to offend God, we see our behavior in better proportion.
(2) There has been a tremendous trivializing of sin in the modern day, including in the Church. That is separate from the corresponding and reinforcing situation of so few confession opportunities announced, realized, acted upon. When sin in general is reduced to pathology (out of our control) or stupidity (errors in judgment), then specific sin has much less weight, and consequently the gravity of any particular sin is also reduced, by derivation.
(3) Many Catholics believe that for sin to be mortal (grave), it practically has to rise to the level of criminality (murder, assault, battery, domestic abuse, drunk driving), or acts mentioned by name in The Commandments (adultery, idolatry, etc.). But causing significant scandal to dependents, or others over which one has authority, is grave. So is detraction (which is a far more frequent sin in the modern day than most people recognize). So is verbal violence. So is harboring hatred. So are so many everyday, habitual behaviors that people, including Catholics, regularly engage in.
(4) If you’re sincerely trying to live in harmony with all of God’s laws – the virtues as well as the avoidances – you (we

) will be less and less inclined to view your sins as categories of ‘mortal’ and ‘venial.’ You will begin to see almost all sin as “serious.” That is, unless you are in a state of lukewarmness/indifference/ingratitude such as Jesus described to Faustyna (which “wounds his heart the most”); in that case you’ll be looking at your sins legalistically instead of relationally.