Not a teacher but my suggestions:
'Quitters never win and winners never quit.'
'If at first you don't succeed, try, try again.' - A slight alteration on one suggested before.
[FONT=georgia, bookman old style, palatino linotype, book antiqua, palatino, trebuchet ms, helvetica, garamond, sans-serif, arial, verdana, avante garde, century gothic, comic sans ms, times, times new roman, serif]'Success consists of going from failure to failure without loss of enthusiasm[/FONT].'
Hope that helps.