Say little and do much." - Mishna, Ethics of our Fathers.
Our Rabbis teach us, and we teach our children not to make too many promises. We are warned that we should do the right thing but never promise. If we promise and fail to deliver, we carry with us not just the fact that we didn't accomplish something good but also a severe sin of commission.
However, there is another lesson for us to learn here. On the one hand, we ought to be sparing in making promises. As much as possible, we should deliver without promising in advance. But if we do guarantee, we must make good. We should guard our words, not just that we don't say something we didn't intend, but also that everything we say will be true in the end.
Rabbi Aharon Lichtenstein used to say that our ability to communicate verbally is what lies behind our humanity. When humanity was created, we were made in the divine image. Onkelos tells us that the essential part of the divine image is the ability to speak. As such, we are charged to use that divine power that G-d imbued us with for good. In practice, this means that we must always tell the truth and that we must always ensure that what we say is true, even ex post facto. We are enjoined to deliver on our promises because we render them true.
This is correct also concerning the promises we make to ourselves. As we approach the new year, it is common and appropriate for us to take on resolutions and attempt to reform for the better. We must take these promises that we make ourselves seriously. Let us all make sure that we both resolve to change in ways that we can deliver, and that we fulfil those commitments.