This is an excerpt of lyrics from Indigo la End's song 彼女の相談. I can't figure out what function the や in line 3 performs.
Does anyone have any input?
や at the end of a sentence has various meanings, but it's roughly categorized into two.
Perhaps you'll see Kansai-ben's や more often, and it's discussed in many existing questions here:
But since や in your example comes right after ない, it's a sentence-end particle. (You cannot say ないだ, right?) It has several functions.
すごいや！ Wow, it's great!
ひどいや！ You're so cruel!
飲めや。 Drink it.
死ねや！ You, die!
シンデレラや、舞踏会に行きたいのかい？ Cinderella, do you want to go to the ball?
それでいいや。 Well, that's okay (if not ideal).
どうでもいいや。 Whatever. / Let it go. / I don't care.
もう知らないや。 I don't care about it anymore.
うーん、わかんないや。 Hmm, I don't know. / I give up.
In your case, や is used in the fourth sense. This や expresses the speaker is not particularly serious about 覚えていない(こと). In the lyrics, the woman is serious, but the guy doesn't even remember what they talked about yesterday.