You need to get past the scruples.
Lesson #1 for a scrupulous person ... do not be paralyzed by "doubt". If you were "dead certain" that these men are not popes AND you reject the reasoning of the (radical) Dimonds, who argue that you can still assist at
una cuм Masses, THEN your conscience would prevent you from attending an
una cuм Mass.
This matter is objectively in doubt and in dispute among Catholics, so it's not just you doubting.
You MAY therefore act in the state of a doubtful conscience without sin.
General Confessions can also be a problem for the scrupulous and are usually not recommended for them ... except perhaps once if it's KNOWN for certain that there had been bad confessions.
I suffered with scruples for a while myself, but I was instantly cured by my spiritual director at the seminary. He told me that unless I could swear to him before God that I had committed a mortal sin, then I was to treat it as a non-sin or venial at most, not confess it, and go to Communion. Then he said, since you're doing this out of obedience, God will not hold you accountable in case you make a mistake of bad judgment. I have never suffered from scruples from that very moment.
Unless you are certain that attending an
una cuм Mass is sinful, just let it go ... and do whatever works for the benefit of your soul.
Finally, read this on the
una cuм issue. You won't find more dogmatic sedevacantists than the Dimonds, and they argue that there's no issue with attending an
una cuм Mass even if an Anti-Pope is being named, and they back up their opinion.
https://www.mostholyfamilymonastery.com/catholicchurch/una-cuм-mass/