If the person intentionally withholds a sin then I am not too certain about abosolution. However in ANY confession forgotten sins are forgiven…even if it was a mortal sin that you forgot. When I returned to the church after over a decade of being like paul I went to confession. I told the priest that it had been 18 years since confession but I did not tell him I was making a “general” confession. I had no idea that such a thing existed at the time. I will tell you it was very very hard to drag myself to the confessional but I chose to do so face to face. when I walked into the confessional and looked up and the priest was my cousin I just about passed out. But It turned out to be a very uplifting experience AND I was walking on air when I left that confessional.