Be strong mentally and spiritually.
Never speak harshly under the fire of jealousy. The mouth can be like a cannon, and speech more damaging than any exploding shell. Be discriminate in your use of words. People do not like to be told their faults. If guidance or constructive criticism isn't welcome, withhold your words. Otherwise, the more you say, the worse you may make the situation.
Thoughts can sometimes be more effective than words. The human mind is the most powerful broadcasting machine there is. If you constantly broadcast positive thoughts with love, those thoughts will have an effect on others. (Similarly, if you broadcast jealousy or hatred, others receive those thoughts and respond accordingly).
Ask God to put His power behind your efforts. If, for instance, it is the husband that is going astray, the wife should pray to God: "Lord, helpme to help my husband. Keep all taint of jealousy and resentment out of my heart. I only pray that he realize his error and change. Lord, be with him; and bless me that I do my part."
If your communion with God is deep, you will see that person change. The more errant a person is, the greater kindness you should give. Instead of succumbing to jealousy and fear of losing a loved one, strive for right attitude and behavior, keep yourself physically attractive, and be strong mentally and spiritually.