You decided that she wasn't the right person for you, did you/didn't you?
Should it be did you or didn't you here? And why?
For questions where you expect a particular response, you use the positive or negative version of the tag question depending on the main verb and use the opposite form to express the alternative possibility.
Here, the short version of the question is: "You decided that, didn't you?" Adding the extra words would not change the structure, so you would say: "You decided she wasn't the right person for you, didn't you?" This form expresses that you expect the answer to be "yes."
For rhetorical challenge questions, you use the same polarity. The short version is: "You decided that, did you?" The long version is: "You decided she wasn't the right person for you, did you?" This structure is usually a rhetorical question expressing your surprise at the choice and your expectation that bad consequences will follow. You don't expect a yes or no answer to this, but maybe anticipate a counter argument or explanation for why the choice was made.
The use of did you would imply that the questioner was seeking an answer that might be either positive or negative.
The use of didn't you implies that the questioner is merely looking for confirmation that you did - or, in some situations - especially those involving court cases or interrogation - putting pressure on you to admit that you did.
I think you might be asking because there's a negation in the sentence ("wasn't"). This doesn't wind up changing the answer, but negations can. These endings can affect meaning and tone, but the rules change with different sentences.
You closed the door, didn't you?
Here the main verb is positive. This situation has been well covered in other answers: it looks for confirmation. You could understand this "didn't you" as being the same as "right?":
You closed the door, right?
You closed the door, did you?
This is very different in tone. It suggests a skeptical, aggressive tone—"Ohh, so you closed the door, did you? Oh really?" The only thing that this "did you" brings to the meaning is emphasis; you could think of replacing it simply with a question mark ("You closed the door?")
You didn't close the door, did you?
Here the main verb is negated ("didn't"). This turns things on their head, because now we're back to the meaning that looks for confirmation, and can be replaced with "right?".
You didn't close the door, didn't you?
Just to cover all the bases: This one is nonsensical and doesn't work (because double-negatives aren't often a thing in English).
But your example sentence should still take "didn't" if you intend simple confirmation... because the main verb is not "wasn't" but "decided."
You decided... didn't you?