Here's where your mistake comes in. Read carefully St. Paul when he says in 2nd Thessalonians 2:15, "So then, brothers, stand firm and hold to the teachings we passed on to you, whether by word of mouth or by letter."
We believe St. Paul -- Scripture is not the only "teachings" of the Church, "word of mouth" is equally part of the teachings, and in fact was given first place by St. Paul. And that word of mouth was preserved by the Catholic Church for centuries. In fact, Scripture as eventually codified was the result of the teachings of the Church as to what would be left in and what would be left out.
I presume that your reference to "adding to God's Word" is taken from Rev. 22:18. Of course, when St. John wrote the Revelation, he was referring to the biblion or scroll of Revelation, not to the entire Bible . . . which didn't exist yet.
So please don't presume to tell us that we're headed for Hell simply because we follow the teachings of St. Paul . . . . as dear old Noll once said, "I beseech you, in the bowels of Christ, think it possible you may be mistaken."
Perhaps you're right. Where in The Bible do I find dear old Noll's admonition?