Paragraph 1735 of the Catechism distinguishes grave matter from mortal sin. There are a lot of vague reasons why we may not be culpable or less culpable than for mortal sin.
I have not gotten a complete answer from any priest, bishop, or the USCCB about how to apply 1735.
In the category of psychological reasons for diminished culpability, one priest said that that narrowly means criminal insanity. I object to that on a couple grounds. One, there is a shifting definition of criminal insanity in the U.S. and in countries around the world. There is no one definition of it. second, I object to that because if the bishops of the world wanted to say that, they could have said it was restricted to criminal insanity. Third, I object to it because it appeals to criminal insanity which is defined OUTSIDE of the Catechism. So, it other words, on those terms, it says really nothing.
I think the words of 1735 should mean what they mean in the ordinary sense in English, until proven otherwise. Psychological factors could point to mental stress, the stress of having a disease, to the lack of maturity, etc. Now, scripture says we are not given any temptation that we cannot overcome with God’s grace. but, 1735 gives us a list of reasons why we many not not only succumb to sin, but be less culpable or not culpable for committing what the Church otherwise defines as grave matter.
1735 says that Grave matter does not automatically equate with mortal sin. I believe that is why it uses the term grave matter instead of plainly calling it mortal sin. Grave matter MAY be mortal sin, except for 1735.
1735 says that habit may lessen our culpability for a sin. That same priest says quickly, yes, but we are guilty of forming the habit and not trying to eliminate the habit.
1735 distinguishes (I forget the word) unintentional acts. So, that must mean that the other categories are actually actions that may be been committed intentionally.
Social pressure: a teen might be pressured into stealing to conform to a social group. If that is the only reason for the action, his culpability may be less. Maybe a woman has an abortion due to pressure from her husband, family, boss, or boyfriend, against her free will.
“Other factors” – you tell me.
If 1735 means anything, then we shouldn’t have to confess this grave matter, either. Otherwise, what practical purpose is there in stating this? Remember, it is a part of the deposit of faith. We should not be bullied into rejecting it. I’ve never had a priest ask me if anything I confessed was excluded as a mortal sin by 1735, so as to reassure me that that act was not a mortal sin.