When your feeling ain't the same and your body don't want to
But you know gotta let it go cause the party ain't jumpin' like it used to
Even though this might bruise you
Let it burn
Let it burn
Gotta let it burn

from Usher – Burn Lyrics on Rap Genius

Meaning

When you know you don’t feel the same as you used to and you know you gotta tell that person how you feel it’s gonna hurt but you know you got to because if you don’t it may hurt you or that person worse in the long run
And when he said “the party ain’t jumpin like it used to” that means like when you at a party and the party was hype you wanna stay.. but when it starts gettin lame you wanna leave because there is somethin else you can do. So he it comparing a party to their relationship.

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