First things first, watch this video:
Check the way they use paint here. He starts out clean (naked, vulnerable), but as his anger and bitterness grows, the lines start to appear, first around him and then on him. The paint fills in to cover him up, and he wears his unhappiness like fitted clothing. The sharp angles and different colors blend him into the wall so that he himself becomes hard to distinguish from the world around him. He is losing himself to the break up.
Then she comes in. She starts already dissected by the paint, but as she acknowledges her pain, the paint disappears. For her, she lost herself during the course of the relationship (“Now and then I think of all the times you screwed me over/but had me believing it was always something that I’d done”). Her resolution to stop living in guilt for things that he had done releases her. She sheds a single, barely visible tear before the paint begins to go.
Both he and she appear naked in the video. For him, this nakedness symbolizes his lingering vulnerability to her. As he progresses through his emotions, he comes to the conclusion that she is just somebody that he used to know, and he becomes closed to her. In deciding this, he also rejects something that is essentially him. He lets himself be lost to his anger. He begins clean and ends broken and covered.
She begins covered and ends naked. When the paint starts to disappear from her body, her negative attachment to him also disappears. The way he treated her (“I told myself that you were right for me/but felt so lonely in your company”) caused her to lose her vulnerability, and without him, she has the chance to start again.