When you say it's breaking the cards, I assume you mean that the part of the card that protrudes above the stitching of each layer (basically, along the swipe strip). I don't own this particular wallet, but I really don't think its this particular wallet's fault; I've experienced this problem with numerous wallets of similar design.
Three ways to resolve the issue:
1. Carry it in your front pocket; or
2. Put more than one or two cards in each slot; or
3. Purchase a wallet without individual card slots (my current wallet):