You need to talk with a copyright lawyer about your problem. She/he is likely to tell you that you own the copyright on your works unless there is a specific "

You need to talk with a copyright lawyer about your problem. She/he is likely to tell you that you own the copyright on your works unless there is a specific "work for hire" provision in your contract with the client; the specific contract provisions regarding ownership of intellectual propery are key. Copyright notices, by the way, are no longer required.