Everyone is afraid of losing closed ones but that shouldn’t affect us negatively.
Try to believe in yourself and invest in relationships that you want to keep. At the same time don’t be too much dependent on them.
Being insecure is being okey nowadays, but our secure doesn't depend on someone. Never think of losing people we're all gonna lose someone someday, so stop bothering youself and start creating wonderful memories with the people whom you're afraid of loosing. Just smile and create memories.