You take your dress to the tailor to be altered. He tells you he can't alter it. The dress is unalterable. Something unalterable cannot be changed.
Take the prefix un-, meaning "not". Add it to the word alter, meaning "change." Tack on the suffix -able, meaning "possible." The result? Unalterable, "impossible to change." The things you've done in the past are unalterable. The only things you have the power to change are the things you're doing right now or that you will do in the future. How can you alter things that haven't happened yet? Good question. For all we know, they're unalterable too.