The speaker keeps making brave promises and then catching themselves in the lie. Four times they say something confident, such as "I'm not scared anymore" or "my heart is strong," and each time the next line undoes it with a blunt "But." The repetition makes it feel like a habit: say the tough thing, fail to live up to it, admit the truth.
In the second half, the speaker turns to the split between brain and heart. They wish the brain could teach the heart, and they plead with the heart to listen to reason. The last lines give up on that hope, calling both of them "useless stupids." That harsh phrase sounds more tired and frustrated than angry.
The mood is raw, honest and a little defeated. The simple, plain wording, with its small grammar slips, makes it feel like a private confession. The "life lesson" of the title seems to be that knowing better doesn't stop you from hurting or repeating mistakes.