Mortal sin... how do you, or anyone else (including the Pope) know that it's mortal? Only God is the judge here, and he's also far more powerful than the devil!
"If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained." (John 20:23, RSV)
It is the Holy Spirit which guided the true Successors of Peter, Prince of the Apostles, which meant guiding them as to what is (and is not) sinful. It is only in these Last Days that, like Pope Honorius I, has the claimants to the See of Peter fallen into apostasy and heresy. Is such scandalous? Yes, absolutely; however, the universal Ordinary Magisterium is absolutely immutable, coming as it did from the immutable One and Triune God through His One and Only Son, Jesus Christ, a Perfect Being.
You have no one to guide you, Nick, except your Orthodox bishop who cannot agree with his fellow bishops on what is fundamentally true. And, then, there are the Coptic Christians. They hold both Roman Catholics and the Eastern Orthodox to be in error. So, why should we believe you other them?
These divisions are not, in my opinion, what the Holy Spirit willed (or wills) for the One True Church, and there is absolutely nothing that you could ever say to me that would convince me that Eastern Orthodoxy is the religion which Christ willed. I can provide you with a plethora of quotes from Saint Augustine and those Fathers before him which asserted the Primacy of the Roman Pontiff, and you will not be able to provide a single quote from anyone who took Saint Augustine to task for his claims. At a very minimum, asserting the Primacy of the Bishop of Rome (or, at least the chair) is neither heretical nor false, and the
alternative, well, there is none, at least as far as I am concerned. What you (and, the Coptics, too) propose is a Church without a head, a view no different from Protestantism with its plethora of "private interpretations." Even sedes assert the immutability and primacy of the Ordinary Magisterium of the Catholic Church, which no Pope is free to change, but obviously, free to deny, as scandalous as that may be.