The speaker is looking back on a relationship that hurt them and explaining why they ended it, or at least ended their hold on it. The opening lines are honest and a little conflicted: they didn't want to get over this person, but they had to. The ex is described as "a poison to my happiness," and the lies and walking away are what wore the speaker down.
The turning point comes in the middle. The speaker used to believe the problem was the other person, but now says "it wasnt you but it was me." That doesn't read as simple self-blame. It sounds more like the speaker realising they had stopped being themselves while chasing something they couldn't get.
The mood is tired but determined. The short, plain lines and the blunt ending, "So I did," make the decision feel quiet and final, as if the speaker is relieved more than triumphant.