Hello, I'd like to help you with your question.
It can be very difficult to break a belief that you have based on proven past history. If you felt you had a lot of bad luck after breaking a mirror and society backs up that belief with superstitions, you can develop a thought pattern that is hard to break.
It may help to try two things. One, change how you think about these discs. That means seeing them only as discs, regardless of their properties. For example, say to yourself "Yes the are shiny and reflective, but they are only discs, not mirrors". Keep facing these beliefs and countering them with realistic thoughts.
Secondly, try challenging the belief itself. Mirrors, no matter how many times you break them, do not cause bad luck. It is only a belief based on superstition. It helps to look up the origin of the suspicion so you see why it is there and the irrational beliefs behind it. Also, use logic. If broken mirrors really did
cause bad luck, wouldn't people do something about that by making unbreakable mirrors?
If it helps, write down your thoughts and the challenges you come up with. Print out evidence against bad luck you find on the internet. Keep reading and rereading your papers. It can help the new ideas/beliefs to stick.
If this doesn't work for you, consider talking to a therapist. You may need the support to help you work through this issue.
I hope this has helped you,