Nothing for it but to analyze your melody and try to identify all its sources, even unconsciously copied sources. If you correctly identify all your sources, and they turn out all to be public-domain sources, you're in the clear. As long, that is, as you don't fall afoul of anyone's trademarks.

You might also join the crusade for a shorter duration of copyright. If we had a shorter term, the chances would increase that any one instance of unconscious copying would not be infringing.