What is called a promise in Buddhism?
Talia, there is no 'sin' in breaking a promise made with pure intentions of keeping it. (In fact, if someone promises with all his heart to kill me, I hope he breaks it!)
Even for monastics, like me, we occasionally have to break our promise or agreement to do something (such as accepting a invitation to a meal or to give a talk) when something else more important crops up. It would be unwise to think we have to keep all our promises, made based on circumstances at that time.
It's only wrong if we 'promise' with the intention of not keeping it. In this case, the so-called promise is actually a deliberate lie.
Can you actually be sure that you wouldn't do something in the future? I can't. I know my mind now, but I can't know how it will be in the future. So, how can we promise what we cannot be sure of?