This is an interesting question. I'm honestly not sure I have ever come to the conclusion that the New Mass is invalid, per se.
I first questioned the validity of the English translation because the form of the sacrament, in every single Magisterial docuмent and in Montini's Missale Romanum, is very clearly established to be more than simply, "This is my body, This is my blood." There are numerous theologians who suggest that this is all that is necessary, but there is not one--and I repeat, there is not one magisterial docuмent that says this and, in fact, every docuмent that says what the form of the sacrament is tells us that the form is more than those simple phrases. Thus, when I discovered the form of the sacrament uses the word "many" instead of "all", I immediately began to question its validity. I began to question the validity of the English Mass about 20 years ago, long before I even knew the traditional Mass was still be said anywhere.
Then I read Archbishop Lefebvre's Open Letter to Confused Catholics in which I learned that there is more than valid matter and form required for the valid celebration of the sacraments. This is where I first learned that the intention of the celebrant must also conform to what the Church intends in order for the sacrament to be valid. From the conversations I had had with priests and by listening to their homilies I had begun to realize that a great many of them did not see the Mass as a sacrificial function but rather as a meal celebrating the fellowship of believers.
Still later the SSPX published an article in defense of the new rite of episcopal consecration and I learned, for the first time, that there has been a debate concerning whether the bishops of the Conciliar church were even valid bishops when consecrated in the new rite and that this debate began almost immediately from the promulgation of the new rite. Thus, when I finally came to the conclusion that the SSPX's defense was too weak and that it was much more probable that the new bishops, and therefore, the priests they ordained, were simply not validly ordained clergy who could even offer the Mass--whether it be the Novus Ordo or the traditional Mass.
It was even later that I came to the conclusion that the sedevacantist thesis is correct and that the New Mass was actually established by heresiarchs and, therefore, were probably intended from the beginning to be a Protestant worship service--and I discover that this is precisely what Montini had envisioned from the outset.
So...This was how it came about that I determined that the New Mass is invalid.