What would you do if someone broke their promise to you

I am used to those circumstances. It's rare if they don't break them.

I just see the fickle nature and feel sad or don't care

I’m loyal as fuck until you lose my trust. When that becomes the case you can forget you ever knew me.

If they do not have the courtesy to tell me why they broke it, on their own, I may or may not ask them...
If there is a good reason, well and good.
If they just waffle around, their word becomes useless to me.

I'm used to that.

Depends on the promise. A big one might make me push them out of my life. Something small can be overlooked or discussed.

Never give them the opportunity to promise me anything again

I'd get over it but I wouldn't forget it.

It depends on the circumstances. No one is perfect, and sometimes unforeseeable complications arise. I might overlook it or it might count as a strike.

Depends on who, what promise they broke

I forgive, but don’t forget.

I’d remember it.

I won’t ever forget it