Truthfulness is more than not lying. It is a way of living in which what we say, what we do and what we believe line up well enough that others can trust us. It also includes honesty with ourselves. Many people find it harder than expected, because truth can cost comfort, approval or advantage, and because careless truth can hurt.
The Gita gives truth a central place. Gita 16.2 lists harmlessness, truthfulness, calm without anger and compassion among the divine qualities, and Gita 17.15 describes disciplined speech as words that do not hurt, and that are truthful, agreeable and helpful, so before speaking ask whether your words are true, kind and useful. Gita 17.26 explains that Sat signifies what is real and good, so align your choices with truth and let integrity, not convenience, decide what you do.
The Gita also links truth with character and reputation. Gita 2.34 says that failing in duty damages one's reputation and character in a way that outlasts the moment of discomfort, so choose actions you can stand by in the long run. Gita 10.4 says qualities such as truth and self-control arise from the divine source, so recognize your good qualities as gifts to be used with gratitude. Gita 17.28 says that acts without conviction come to little, and Gita 16.7 shows how, when discernment fades, purity and truth are quickly lost.