What happens to Catholics if they forget to confess a sin?
Sins inadvertently forgotten after a careful examination of conscience are included in the absolution.
answers.encyclopedia.com/question/happens-catholics-if-they-forget-confess-sin-136171.html
Question: Is a Confession worthy if, without our fault, we forget to confess a mortal sin?
Answer: If without our fault we forget to confess a mortal sin, our Confession is worthy, and the sin is forgiven; but it must be told in Confession if it again comes to our mind.
catholicism.about.com/od/baltimorecatechism/f/Question_215_BC.htm
God bless you, and welcome back to this wonderful Sacrament!