When you confess, you only need to list kind and number, and if you don’t know the exact number, you can give an approximation or average.
So you don’t have to tell the priest your biography, which I can understand might be overwhelming for you, just I got roaring drunk each weekend for 7 years , or I robbed banks three times and shoplifted around 4 times a week for 6 months. (Totally making those up.)
Also add any factors that make it worse: I burglarized places 6 times, and one of them was a church and I took the money for the poor.
If you go to confession with a sincere desire to confess all your mortal sins, they will all be forgiven, along with the venial sins.
If you withhold just one mortal sin
deliberately, say from embarrassment, then that confession is sacrilegious and all those sins will have to be reconfessed, as well as any sins from subsequent confessions, and the sin of sacrilege for each sacrament received thereafter. (I made a sacrilegious confession last year, so received the Eucharist sacrilegiously 50 times, and confession 12 times. Here is the sin I did not confess, and here all all the others.)
I am telling you this so you won’t worry about forgetting a sin or leaving something out in case of nervousness or because you didn’t know it was a sin. If that happens, it’s not a problem.
If the recollection of a sin comes to you later, just say it at the end of your next confession, like this: Father, for these and all the sins of my past life, including X, I am mostly heartily sorry.
Congratulations on accepting God’s grace and turning your life around! And welcome back
