Indeed.  Great quote from St Augustine and good point by you, regarding Church membership.

St. Augustine didn't have the protestants to contend with (1500-),

He had the pelagian heretics to deal with.  They were baptized catholics, though they started to deny the necessity of baptism (just like many protestants today).  The point is, even if you are baptized, your heresy cuts you off from the Church's membership.  This principle still stands today.

I ask, because I constantly have to fight myself in knowing whether God is unhappy with me in attending a church that is not recognized by Rome.

Eternal Rome > modern rome.

The Arian heresy in the 4th century engulfed 95% of catholics in that day...St Athanasius stayed strong to doctrine and truth.  Today is no different.

St. Athanasius, pray for us.

How does this quote apply to today's times?

St. Augustine didn't have the protestants to contend with (1500-), nor the Crisis of the Catholic Church (1962-), nor all of the different Catholic opinions currently (1970s-)?  People wanted to follow Christ, so they became a Christian (Catholic) and then the rules were straight forward without any confusion.

I ask, because I constantly have to fight myself in knowing whether God is unhappy with me in attending a church that is not recognized by Rome.  Rome would say that we are in schism and have cut ourselves off from the Body of Christ.  We would say that Rome cut itself off by not practicing True Catholicism. 

What if we are not really pleasing to God?  What if we have it all wrong?  What if we are suppose to close down our churches and return to Rome?  What if .....

I know I am babbling and I know I will just keep doing what I am doing and hoping that God has mercy, but I still long for a simple time.  Which probably never really existed.  I wouldn't have wanted to be fed to the lions, either.

If you believe, in good faith, to the best of your ability, that "Rome" is not practicing True Catholicism, then you have an obligation to separate yourself from that false "Rome."

That false "Rome" is the "Babylon" that we are told to "go out from" (Apocalypse 18):

1 And after these things, I saw another angel come down from heaven, having great power: and the earth was enlightened with his glory. 2 And he cried out with a strong voice, saying: Babylon the great is fallen, is fallen; and is become the habitation of devils, and the hold of every unclean spirit, and the hold of every unclean and hateful bird: 3 Because all nations have drunk of the wine of the wrath of her fornication; and the kings of the earth have committed fornication with her; and the merchants of the earth have been made rich by the power of her delicacies.  4 And I heard another voice from heaven, saying: Go out from her, my people; that you be not partakers of her sins, and that you receive not of her plagues.  5 For her sins have reached unto heaven, and the Lord hath remembered her iniquities.

Meditate and pray about what is being commanded by the Angel in Apocalypse 18:

1. The Angel from heaven calls us his people. He is talking to the faithful remnant of the Roman Catholic Church.

2. He says to "go out from her." What are we to go "out" from? We are to "go out from" the false Church so that we "be not partakers in her sins."

3. What does the word "partakers" in her sins imply? That we not be in "communion" with such a detestable abomination. We are required to break "communion" with the "habitation of devils." Otherwise, we condone their sins.

4. When we give our consent to the sins of the false Church by remaining "in communion with it," we will receive the punishment of the "operation of error" (2 Thessalonians 2). Instead, the proper response is break communion and "hold the traditions."

10 And in all seduction of iniquity to them that perish; because they receive not the love of the truth, that they might be saved. Therefore God shall send them the operation of error, to believe lying: 11 That all may be judged who have not believed the truth, but have consented to iniquity. 12 But we ought to give thanks to God always for you, brethren, beloved of God, for that God hath chosen you firstfruits unto salvation, in sanctification of the spirit, and faith of the truth: 13 Whereunto also he hath called you by our gospel, unto the purchasing of the glory of our Lord Jesus Christ. 14 Therefore, brethren, stand fast; and hold the traditions which you have learned, whether by word, or by our epistle.

He does not say "heresy cuts you off..."

  He asks, "And if a member be cut off..."  Can a member (i.e. hand, foot, finger) cut itself off?  No, a person does such, by sin.

So, logically, how is a member "cut off" from the Church?  Answer:  Heresy or schism.  Therefore, heresy/schism cuts one off from the Church (i.e. membership).

Yes, yes, Fr Wathen said, "once a catholic, always a catholic" but he was speaking of POTENTIAL membership.  No baptized person loses potential membership.  But Heresy/schism concern ACTUAL membership (i.e. current status).  And such sins (and the accompanying excommunications) put one in a state of spiritual 'time out'.  Potentially, the person can get out of the 'time out/suspension' anytime they want (i.e. repentance), but until they do such, they are *treated* as being outside the church.  They can get back in the church anytime they want (an unbaptized person is not part of the church, in any way), but if there's no repentance, there's no membership.

Analogy:  A person who passes his driver's license test is a legal driver.  But they could lose their license (it's suspended) if they repeatedly violate certain traffic laws...even though they are STILL a legal driver.  They can get their license re-instated if they pay the fines and admit guilt.  If they don't, then their status as a 'legal driver' remains suspended.

You have to read it like he taught it and not misapply it to sedeism, which did not exist when he wrote it.

Yes, yes, Fr Wathen said, "once a catholic, always a catholic" but he was speaking of POTENTIAL membership.  No baptized person loses potential membership.  But Heresy/schism concern ACTUAL membership (i.e. current status).  And such sins (and the accompanying excommunications) put one in a state of spiritual 'time out'.  Potentially, the person can get out of the 'time out/suspension' anytime they want (i.e. repentance), but until they do such, they are *treated* as being outside the church.  They can get back in the church anytime they want (an unbaptized person is not part of the church, in any way), but if there's no repentance, there's no membership.

Analogy:  A person who passes his driver's license test is a legal driver.  But they could lose their license (it's suspended) if they repeatedly violate certain traffic laws...even though they are STILL a legal driver.  They can get their license re-instated if they pay the fines and admit guilt.  If they don't, then their status as a 'legal driver' remains suspended.