Jealousy is an emotion, not proof of devotion and not a reliable guide to what another person has done. It can be useful to understand what is underneath it while staying responsible for your actions.
Name the feeling and the story
Try separating an observation (“they made plans with a friend”) from an interpretation (“they will leave me”). The interpretation may feel convincing without being confirmed.
Choose a response that respects both people
It is fair to ask for reassurance or talk about agreements. It is not fair to monitor someone's phone, isolate them, threaten them or demand that they give up ordinary friendships. Those actions restrict another person's freedom.
Look for recurring patterns
If jealousy keeps coming up, ask what might be underneath it. A trusted person or counsellor can help you work that out without shaming you.
When jealousy becomes control
If someone checks your phone, limits your friendships or makes you afraid to disagree, that is more than an uncomfortable feeling. Talk to someone you trust or contact a local support service.