Am I sinful if I leave my stubborn, hard-hearted wife in a state of suspension after a marriage that has lasted 18 years, due to her excessive slanders?

If the wife is in the aforementioned condition, this contradicts good companionship. Therefore, you must advise her and remind her of her bad conduct. If she does not reform, then it is better to divorce her, because the continuation of the marriage may become a pure corruption. It is not permissible to keep her in suspense [neither married nor divorced], but rather either retain her honorably or release her kindly, unless that is with her consent and her waiver of some of her rights.