My last confession I had a list on my email of things I wanted to confess, and I said the things I was most ashamed of but I left out some things that I didn’t think were mortal, but now I regret it and wish I just confessed them in case they were mortal. My question is… Could I go to the same priest I confessed to and explain this to him, and tell him the things I left out to see if those were mortal or not ? Or would I have to confess everything that I previously confessed also?
Can you discern whether they are mortal? Were they grave matter? (If you don’t know, check the Catechism. Sins that are grave matter fall under one of the Ten Commandments.) Did you do them voluntarily and knowingly? If the answer to both questions is yes, then they were mortal.
It is a good idea to confess venial sins too, though.
Yes, and no.
You didn’t knowingly withhold mortal sins qua mortal in confession. You withheld sins, but did not withhold sins that you knew you ought to have confessed. As such, the withholding was not sinful.
However, I’m honestly not sure whether, having found that some of them were mortal (if you do find that out), you would be obliged to confess again. The Catechism says, “All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are the most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously than those which are committed openly.” You were conscious of those sins, but not conscious of whether they were mortal.
I would err on the side of confessing them, because you probably should learn to make “a diligent self-examination” and discern which of your sins are mortal. If those sins you omitted were moral and you ought to have realized that in your examination of conscience, then you would be vincibly ignorant in omitting them. But I can’t speak specifically because I don’t know your sins.
I would follow your plan, unless you have a spiritual director who has told you otherwise, in which case you really should not be soliciting advice on here but talking to him/her.
I’m wondering how these sins were mortal if you weren’t sure. I would ask the priest, but it seems on the surface hard to accept that one could fall into mortal as easily as you seem to imply.
To quote Senior apologist Jimmy Akin “when in doubt confess.” He does however caveat that for those who are scrupulous who need a spiritual director.