I’m a scrupulous teenager, and I went to confession on Saturday, and I’m afraid I made an invalid confession. I committed this sin on Saturday morning (keep in mind I went to confession Saturday evening) that I believe is mortal now that i look back on it. I knew it was serious, but I had no idea it was mortal. So, since I knew it was serious and it was a grave matter, and I consented to do it, it was a mortal sin. However, I believed it only to be venial, and so I deliberately chose to not say it in confession because I already had a list of mortal sins and I didn’t want to make my confession too long, so I kept out the sins I didn’t think were mortal, including the sin I’m talking about. I truly believed I was confessing all of my mortal sins in confession, yet I deliberately left out a mortal sin (I deliberately left it out, I just didn’t know it was mortal). I know I need to go to confession again to confess the one sin, but do I need to confess all of the sins in my past confession too, or was it valid, since I wasn’t meaning to deceive anyone?
You have to reflect on yourself. Did you believe the sin to be mortal at the time of confession or not? If you did not and only realized later, then you did not act with any malice in witholding a mortal sin. It would have been absolved with the other sins you have confessed at that time.
Now if you deliberately witheld a sin you acknowledge as mortal, that is a different case. You would need to confess that sin plus confess to deliberately witholding a mortal sin in confession.
This all boils down to your understanding and intention. If you did not realize at the time of confession that your earlier sin was mortal, then its fine. But if you did, then you not only have an unforgiven sin, you also sinned again by lying in the Sacrament.
And, keep in mind that I’m not sure under my circumstances it was surely mortal for me.
The sin was forgiven during the confession. Next time you go to confession confess it to the Preist and you are all square with God. Nothing to worry about because you thought that all mortal sins were confessed, if you had really forgot to mention one, just mention it next time.