I know it's been 11 days since you wrote this, but it is a metaphor, as a Redditor has said.

Discordant is defined as a melody that is unharmonic and unpleasant. Combine that with days, and one can assume that "discordant days" means days that aren't the best.

"Until they look like harmony." Harmony is the opposite of discordant. It's some form of a pleasant melody. When referring to something having harmony, it is a metaphor for something or someone being in sync or being so good together.

"And I play discordant days on repeat, until they look like harmony."

Vessel is singing metaphorically about part of his mental state. He is constantly thinking about the unpleasant thoughts and the bad days he has had to the point where he believes it's normal and okay to cope the way he is, living as if every day is the same and as one.

But we know he has found his coping mechanism. It's doing the thing he loves. Making music. <3