I did something which I thought was sinful but it wasnt”
For me personally, I would have said “I did something which I thought was a mortal sin at the time of acting, but now know it wasn’t, but also I can’t now remember what it was and I don’t think I’ve ever confessed it” - that’s if
at the time of acting it
was a
mortal sin. I think you need to ask your priest about this in confession.
Otherwise, if it was a
venial matter, then yes, confessing that way would be ok - because venial sins are not obligatory matter for confession - only mortal sins are.
I have to reconfess the ones I remember saying what I did that I thought was sinful?
Again, only if they were grave matter. And the time you are thinking of now, you are absolutely certain is grave matter, committed at that time with full knowledge that it was, and with full consent.
You would have to re-confess the
deliberately unconfessed
MORTAL sin, state you
deliberately withheld it, and then re-confess all
mortal sins committed since the confession when you deliberately left it out.
In your OP you said “And if you realise you didn’t
ages ago…” You could say you committed it when you were 15/20yrs old in addition to my above
suggestion.
Whilst you haven’t shared other details which may be relevant to this discussion - if you’ve been a practicing Catholic and regularly go to confession, and at each confession have always done your best, then I am not sure how one can omit anything really serious without remembering that they
did omit it, and what sin it was.
I’ll just add by way of example - some sin you committed when say 10yrs old, but now after studying you realize it is grave matter, doesn’t make the sin committed when 10 a grave sin ie mortal sin -
back then just because you
now know it is. So no knowledge about the gravity of particular sin you come by now changes the gravity of the sin committed prior to you coming by this knowledge. Simply because for a sin to be mortal, one has to have full knowledge - and if you are only learning
now that it is grave, then this knowledge you have
now, doesn’t change the gravity of a
past sin.