The dicitonary defines cynicism as “an inclination to question whether something will happen or whether it is worthwhile”.
And indeed, many attempts fail, many promises are broken, and even successes are, on a long enough time horizon, impermanent.
But these are just reflections of the scientific laws which govern the world we live in. When you attempt to do something, by definition, there is a chance that the attempt fails. Promises are broken, either because people never intended to stick to them, or because circumstances changed such that it is no longer possible to keep them. And even successful outcomes are soon hopefully superceded such that the earlier foundations which paved the way for better outcomes are eventually forgotten.
However, these are the laws of the world we live in. As such, we cannot change them.
We can only decide whether to be cynical because of these laws or to accept the laws and push forward regardless.