Also, why exactly is it a mortal sin to attend a Protestant service? It's not fulfilling Sunday Obligation obviously, but why can't you attend one of their services ever?

Because it's false worship. It's worship according to a false religion, and the Church has always taught, since the earliest days, that to participate in the worship of a false religion is essentially to accept that religion and deny Catholicism.
The Church calls it 'communicatio in sacris' with heretics or schismatics (literally 'communication in holy things'), and teaches that it's a mortal sin.

Many say that the TLM was promulgated in 1570. It wasn’t. The TLM was developed organically over the centuries. The Missal of St. Pius V is essentially the Missal that was used in the diocese of Rome in the 1400s. St. Pius V simply codified what he found. He did not promulgate a new rite. This is quite simple.

Here is the TLM from the year 1190. Go ahead and see if there are any differences between the Missal of St. Pius V and the TLM of 1190. Hint: there isn’t.

gloria.tv/post/dNL4zJYU3…

"(We) consulted the writing of ancient and approved authors who have bequeathed to us records relating to the said sacred rites, thus restored the Missal itself to the pristine form and rite of the holy Fathers.” - St. Pius V

To say that this is a ‘schismatic stance’ is just strawmanning lol. A totally baseless and nonsensical claim. So is saying that such statements imply that Norvus Ordo sacraments are invalid, which is also absurd.

Enough on QP and the New Mass being illicit for now. I’ve already rested my case.

The first things NOites and Modernists see in their nightmares.

“Where a harlot has set herself up, that place is a brothel. But the synagogue is not only a brothel and a theater; it also is a den of robbers and a lodging for wild beasts...

when God forsakes a people, what hope of salvation is left? When God forsakes a place, that place becomes the dwelling of demons.”

— St John Chrysostom, Homilies Against The Jews

“Is it not strange that those who worship the Crucified keep common festival with those who crucified him? Is it not a sign of folly and the worst madness?”

— St John Chrysostom

English:

“The Verden Massacre was an event in the Saxon Wars that occurred in October 782 of the era of Our Lord. In that month, the great Charlemagne - Emperor of the Franks and father of Europe - ordered 4,500 Saxon pagans to be beheaded and destroyed the pathetic idol they worshiped (the Irminsul, literally a trunk without branches located in the open sky). Thanks to Charlemagne, Europe was reunited and the Saxons embraced the only true faith.

Blessed Charlegmane, Father of Europe, ora pro nobis.”

Perhaps instead you should give up virtue signaling for lent, since it attaches you to the world by seeking appeasement of people who hate Christ. Recognizing the replacement of my people and continued assault on our culture and history is not a luxury.

The greatest trick the devil pulled wasn't convincing the world that he doesn't exist, it was convincing the world that protestantism is Christianity.

In case someone lied to you, here's a reminder:
Vatican II isn't Catholic or valid.

The Current Rules for Fast and Abstinence - And The Traditional Recomendations

https://sspx.org/en/news-events/news/current-rules-fast-and-abstinence-and-traditional-recomendations-64201?utm_source=twitter&utm_medium=social&utm_campaign=tweepsmap-SSPX.org

Evil stuff. Don't think this isn't coming to Europe and the US soon because it is.

https://twitter.com/cnalive/status/1361465324277743620?s=20

Wednesday is Ash Wednesday and the start of Lent.
Prepare yourself.