The Catechism is very human in that it takes into consideration different circumstances people may be in, as such mortal sin isn’t always as clear cut as people may want it to be.
To take the often used example of masturbation: ‘To form an equitable judgment about the subjects’ moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety or other psychological or social factors that lessen, if not even reduce to a minimum, moral culpability.’ CCC, 2352
That doesn’t mean there’s a ‘get out of jail free card’ but it does mean that some sin isn’t always mortal in given circumstances.
Even priests in the confessional may be reluctant to pronounce something as a mortal sin but still advise that it was right to confess and if I’m doubt to refrain from receiving Holy Communion. (On the other hand I was once, worryingly, told that ‘hardly anyone commits mortal sins as you need to be actively rejecting God to do so’).