Trying to fill in someone else's shoes is futile. The person will at best be a shadow of the one who shoes they are trying to fill. It is best to have new shoes (their own) rather than live in the shadow of another
Some people aren't really loyal to you. they are loyal to their need of you. once their need changes (like if they found someone else that would keep them from being lonely who I guess is better than you), so does their loyalty.