I don’t believe it is necessary for you to determine if a sin is mortal or venial. When I confess, I do a simple examination of conscience and list anything grave in number and kind. I do not say, “This one was mortal… and now here’s the venial ones…”
For example, perhaps I have engaged in an action that constitutes grave matter. No question about it, the action was grave matter and I did it.
But perhaps there are some circumstances involved that may or may not decrease my culpability. How can I determine the full level of my culpability?
The answer of course is that I can’t. Only God can accurately weigh the balance of my sin and my culpability. My job is to get to confession and confess anything that constitutes grave matter.
The Catechism can help us in determining what is or is not grave. It can also clarify extenuating circumstances. We must attend mass on Sundays and holy days of obligation, for example, but not when we’re sick or need to attend to infants or the infirm, or cannot physically get to a valid mass, and so on…
But of course, in the end, our job is simple and twofold:
*]Avoid sin in the first place.
*]Confess sins we have committed.
God bless you.