Many who come to the Church from other faiths are in a similar boat. I became Catholic after over fifteen years (including all my teenage years!) as a Baptized non-Catholic Christian. When I went to my first Confession, I can guarantee that I had forgotten things, that my numbers were wrong, and so on…not because of any intent to deceive, but because our human memories are imperfect.
Rest assured, as long as you
intend to Confess all mortal sins, your Confession is valid. All sins – remembered and forgotten – are washed away in the Sacrament, as long as you aren’t intentionally withholding anything.
The best thing to do is provide your best approximation. You are supposed to Confess what you did and how many times you did it to best of your ability, but that doesn’t necessarily mean an exact number. Things like this are perfectly valid if you don’t know or can’t remember an accurate number:
“I X’d between 50 and 60 times.”
“I did X about five times per week over a three year period.”
“I did Y more times than I can remember.”
“I committed the sin of Z many times over a two month period.”
etc. Just try to be as clear and accurate and honest as you can, but you don’t need to provide an exact number if you don’t know it.
If Father needs more detail, he will ask. In fact you should try to keep it direct and short without a lot of detail. When I was going through RCIA, Father told us that a lot of people tend to give him more detail than he needs. His advice was to err on the sparse side (e.g., ‘I committed murder two times’) and let him ask for more detail if he needs it.
God bless you, and welcome back

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