Stand therefore, having fastened on the belt of truth (Eph 6:14)
I'm no expert in Greek, but I do know how to use the resources over at studylight.org! In the Greek having-fastened-on-the-belt is all one word. So the emphasis here is not about some mysterious object, "the belt of truth", but on an action: "belting-ourselves" with truth. So here, as well as in the rest of the passage concerning the armour of God, is not about some metaphysical protection that we have to don to engage in spiritual warfare (found here and nowhere else in the word), but about how we apply the spiritual principles clearly laid out in the rest of the Scriptures.
Another interesting fact from the Greek is that the word for belt, the root for the word used here, is "zone", from which we get the English word... erm... Zone! So we could also understand this instruction as "zone yourself in the truth".
Some limitations are bad. When we limit our faith or our expectations in God, these are limitations we were never meant to have. But other limitations are good. It is good to have proper boundaries. Proverbs says: "A man without self-control is like a city broken into and left without walls." (Pr 25:28). Self-control is setting up good and righteous boundaries for our behaviour. Even God is "limited" in this way: it is impossible for him to lie. Why? Not because it is too difficult for him or beyond is power, but because it is outside the "zone" of his perfect nature.
In this context to belt-ourselves with truth, means to confine ourselves, in thought, in behaviour, and in confession, to the truth. And the ultimate truth, of course, is the truth of God's word.
It's not about being "honest" about how I feel, or how I assess my circumstances (that might lead to a whole other set of confession and behaviour), but to refuse to think of myself or act in a way that is outside of what God declares me to be. That is the truth that I "zone myself" with. So that to use an expression of Keri Jones, "I am framed by the word of God," and anything that comes in (what I receive) and anything that goes out (what I confess) has to pass through the filter of the Word of God and what he declares me to be.