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If it were me, I would re-confess, just as if the NO priest was not a priest regardless of whatever anyone else says about NO ordinations being valid. They brought this doubt on themselves, we did not.

Confessing one's sins to the priest is not meant to be a pleasant experience because you must really humble yourself to do it. The priest (who is also a sinner) is himself edified by a good confession.  

That is exaggerated. There are still Novus Ordo priests around, usually retired, who were ordained before 1968. Very few bishops are still alive who were consecrated before 1968.

Right, but there are fewer and fewer, and so it's likely that OP confessed to one of these types.

This is certainly a complicated question.

I've come to the conclusion that there's some gray area here between positive and negative doubt, between subjective and objective doubt.  If I personally hold that there's positive doubt, but many priests out there hold that there isn't, I feel that the doubt is in some gray area between negative and positive, between subjective and objective ... i.e. since there's disagreement out there among Traditional Catholics, objectively speaking, rendering the doubt (try to follow) ...

subjectively objective but objectively subjective .

In other words, I personally hold that there's objective positive doubt, but objectively speaking there's disagreemnet about it, making it objectively subjective (i.e. it's my opinion that's not shared by others).

While you could just submit to Fr. McFarland's opinion (or that of your SSPX confessor), you personally might continue to have your own doubts, and if you went to a different priest, you might get a different opinion.

In general, the faithful are not obliged to be theologians and can probably simply go with the opinion of their confessor.  So based on objective probabilism (about who's right on this issue), I don't think OP would have a grave obligation to repeat those past Confessions, but it would be conducive to his peace of soul if he did.  So one thing I might recommend as a compromise position would be to just go to certainly-valid priests going forward, and at the end of each confession, just make part of a general confession each time at the end (since it's always permitted to mention past sins), until you've "caught up".  You could break it down either chronologically (when I was 1-10 years old), next time (when I was 11-20), etc. ... or by Commandment.  As Father Alphonsus used to say, even the most eventful general confession should take no more than 10 minutes, since there are only 10 commandments.  You could just mention a sin and a number, without any more detail than that.  Of course, with general confessions, numbers can be hard to come up with, so maybe a frequency of sin.

That's probably what I would recommend to OP, to be at peace, since you've had a confessor tell you there's no doubt and that you can, in the practical order, proceed on his advice, but then for your own peace, just gradually (without feeling a strict obligation), supplement future confessions with a partial general until all put together you'd have effectively made a general confession.

To sum it up, while you might have your opinion, the faithful are not obliged to be theologians (adjudicating the validity of Sacraments or even adjudicating between the opinions of different priests) and can in good conscience accept the advice of a confessor (even if that confessor might, in your opinion, be wrong).  So I would hold that you're not under strict obligation, given your status as lay faithful, to adjudicate the question of validity or to adjudicate between the opinions of different priests, but I would recommend going forward to confess the past sins little by little at the end of each confession going forward until you've "caught" up, so that you could be at complete peace.

OP, I was going to suggest asking your priest, but then realized that that priest probably thinks the New Rite orders are valid.  Do you have access to /Can you contact a Resistance or Sede priest?  If so, I would get their opinion.  I think they would probably recommend some sort of general confession.