How to break a spell you've cast
Breaking the effects of a spell can be difficult but not impossible, however, there are some things that govern this.
1. You have to have been the one that cast the spell. You can not undo another witches work. You can cast another spell on top of what had been done by another person. you can 'counteract" the original spell but will not be able to fully undo it. I am using this example loosely to give you the idea of what I am talking about, but, think of the film Sleeping Beauty. The evil Queen Maleficent casted a spell on Aurora, and the 3 fairies cast spells that lessened the effects of original spell and negated the long term effects of it. But the spell still had to run its course.
2. The length of time that has past must be recent, and the effects of the spell must not be working. Meaning, that if you cast the spell on a monday and by friday you have decided that you don't want that spell working any more you can undo it. But if you start to see changes around you associated with the outcome of the spell or leading up to the spell being done, then you must wait it out. So think of this like riding an elevator to the 12th floor and you decide you want to get off before then. The elevator is still going to go to the 12th floor regardless of you intent and will. Just like if you got on an elevator going down and you wanted to go up, it has to finish going down to then go back up.
Take a moment to view the video for more on this topic as well as a ritual ideas on how to undo your spell work.