Pretending to be happy is the subject here, and the poem treats it as a skill you can practise. It opens with a piece of advice in quotation marks: hide your anger, rehearse fake smiles in the mirror, wear a mask. The speaker then follows that advice through. Once the smile and laugh are convincing enough, even when you're "dying inside," other people start to believe everything is fine.
The feeling underneath is tired and a little bleak. The speaker sounds like someone who has learned that showing real emotion feels risky. The third stanza makes this clear: if you let your guard down, your walls fall and the smile everyone trusted disappears with them. So the closing lines urge you to keep the walls up, "even if you're a mess."
The mood is lonely and quietly sad. The short, plain lines and the rhyme at the end make the advice sound almost like a rule, which makes it feel more heartbreaking. Nobody sees the real person, only the mask.