Is it permissible to reject righteous suitors while awaiting a former suitor with whom marriage will not take place, and is there sin in fearing that the suitor might change after marriage or marry another?

You bear no sin for rejecting a suitor and awaiting the one you desire. Your apprehension about a suitor's character changing after marriage is unfounded, for the failure of a first engagement does not preclude good from being found in another suitor. Make a good choice, then rely upon Allah and think well of Him, and do not pay heed to whispers and delusions.