Am I sinful if my father canceled my engagement to someone I was content with, based on negative testimony from two of his friends, or is this a divine decree that must be accepted?

Usually, a girl's guardian is keen to choose a righteous husband for her, due to his experience and compassion. So, your father's action of inquiring about the young man was correct. And if those who informed him about his situation were trustworthy, then he did well in rejecting him. There is no sin upon you in this, and as long as you have sought Allah's guidance (istikhara) and were sincere in doing so, then be confident that Allah will choose what is good for you and avert evil from you.