EWTN Catholic Q&A
Question from obh on 11-20-2013:

Due to many years of not going to confession , a few grave sins were not mentioned when i finally went . just now I realized that I probably missed some grave sins . My conscience is bothering me now and questioning my state of grace.

Answer by Fr. Jay Toborowsky on 11-20-2013:

If the sins were genuinely forgotten at the time of the Confession, then they were forgiven in the absolution. If they were intentionally not mentioned, then they were not forgiven.