The speaker looks back on a painful time after a relationship ended, or was ending. Just seeing the person they loved hurt, especially when that person seemed happy with someone else. The speaker says it made them sick, and the plain wording makes the pain feel raw and honest.
The most striking moment comes when the other person still tried to make things work. You might expect that to bring comfort, but the speaker calls it the most painful memory, which suggests it kept hope alive while making the situation more confusing. Offering to wait is called "self-torture," as if the speaker was hurting themselves by staying. The last line is cold and final: the other person will never know the speaker's inner thoughts. The mood is sad and tired, with a quiet bitterness, and the short, simple lines make it feel like a private confession that the speaker has decided to keep closed.