Stopping believing in karma hurts because you have to accept life isn't fair, but it's also freeing because karma is a really harmful concept in the end. An example, people saying abusive families, sexual abuse, are because you did something to deserve it, or it's meant to "make you a better person", disgusting romanticization. It also stops people from taking action and holding others & themselves accountable. It's a way for people to cope with the meaningless suffering of existence, but it's not a harmless way to cope. I think these lyrics from HEALTH's song SLAVES OF FEAR says it better than I can: "No pain has purpose / No one above to judge us / No pain has justice / No one above to love us"
You can believe whatever you want but in my opinion I think you should accept it's not real. It's just going to hurt you & others to believe it is. Sometimes I still wish karma was real because I want people who hurt others to not get away with it, I think we're all going to wish that sometimes.