The main thing is to keep the main thing the main thing, and for you the main thing is this: How do I help myself and others be the best they can be? How do I help them through their troubles, not necessarily in the way they want, but in the way that is the best for all involved? How do I encourage others in their state of life and the trials of their circumstances? Between finding “the courage to change the things I can change” and “the serenity to accept the things I cannot”, how do I find the wisdom to know the difference and chart the path through each one? These are the questions to ask.
As for what is a mortal sin and what is not, it is best to strenuously avoid *all *sin of *all *severity, and when it seems one has committed what may be a grave sin to confess it as soon as possible and to avoid sacrilege by refraining from the other sacraments until one can confess. When in doubt, then that’s a person who could benefit from seeing their confessor as soon as possible, for they have committed a sin that is well worth fleeing! If you fear that someone you know may be in mortal sin, then that is someone who ought to be encouraged to renounce their path and turn towards something better. Even if the sin is not mortal in gravity or the person is not culpable, after all, it is still a situation well worth getting out of!