This is the mistake people apologise for most and repeat most. Not because the rule is hard — it is one of the simplest in English — but because the two words sound identical, and the part of your brain that picks words while you type works on sound before spelling. Autocorrect will not save you either. Both are real words, correctly spelled, so nothing gets underlined and nothing gets flagged. The only defence is a test you can run without thinking, and there is a good one.
A possessive determiner. It marks something as belonging to the person you are addressing.
A contraction of "you are." The apostrophe stands in for the missing "a."
Quick way to remember
Read the sentence with "you are" in place of the word. If it still makes sense, you want "you're." If it collapses — "you are application arrived" — you want "your." The test never fails, because "you're" is nothing more than "you are" with a letter removed.