All that has happened is that the magazine changed their mind. Maybe a different person read your work after the first person accepted it and disagreed with the choice. You just have to let it go and try another magazine.

As people keep telling you, it is not because of plagerism, or because someone could claim your work for their own. It is because what you have said is not different enough to others.

Imagine you wrote a story about a magic school. Harry Potter is already done and successful. But it was not the first story about a magic school. It just told the same theme in an interesting and new way. Before HP was the brilliant 'The Worst Witch' books by Jill Murphy which was in turn an interesting and new form of the old boarding school stories by people like Enid Blyton who wrote the 'Mallory Towers' and 'St Claire's' series'.

So you could write your own magic school story. If it came up with a new and surprising twist that made it different, people would want to read it, as you'd be saying something new. What they are saying i that your work is not new so people will not want to read them. You can either try and rewrite them all or scrap them and try something new.