I find no evidence that Shakespeare wrote this. I see no use of “ascertain” in his work, and only three unrelated uses of the word “resolutely.”
The question remains, however – who said it, if Shakespeare didn’t?
How about Aesop, the guy from all the fables?
This is the only reference I can find that suggests Aesop, and even then the context is a little weird – the quote in question stands out in the middle of the page, somewhat unrelated to the rest of the context.