When I say, "I know," I always mean "for myself" - "according to my understanding". I really like the way Nephi phrases that concept, so I borrow from it. "My experiences have taught me personally" is how I define "I know" in this context - so I'm fine in saying things like "I know the Gospel is true" and even "I know the Church is true" - since I define "true" as "on target" (true flight of an arrow) or "pointed in the right direction" (true north) or "sincere" (true love). I have found a way for the Church to be an instrument (a vehicle) in my life, and I have found a way to accept the messiness and infallibility and ambiguity as unavoidable (and sometimes even personally ennobling), so I honestly don't worry much about it from a cosmic standpoint.
Also, I've spent decades studying and viewing other denominations and religions, and I simply haven't found a single alternative that inspires me at the highest level like Mormonism does. I believe we continue to struggle with the effects of centuries of doctrinal bastardization that haven't been pruned fully from our own tree, but I "know" I don't want to construct my personal paradigm anywhere else. It would be MUCH easier in MANY other religions and denominations, but I know I personally wouldn't be lifted and inspired and motivated to grow and "become" like I am in the Church.
So, in summary, what I am left with in the end is nothing more profound or deep than:
I know I want to make my journey within the LDS Church.