Is it okay that I broke a promise to a fiance?
Breaking promises is horrible especially if it's to your fiance.
I don't know if it's horrible or not. I think it could be
You shouldn't have made any promises you know you can't keep. Trust is an important thing in a  relationship or marriage. You should always keep your promises or relationships will eventually fall apart.
I guess so. I'm going to focus on my studies and work hard.
It's good you are trying to make it up to your fiance at least. Win their trust back over and be more careful in the future about what you promise to them.