Theme is the argument your ending makes. It is not a line of dialogue, a motif, or a subject. 'Grief' is a subject. 'Grief only ends when you let someone see you failing' is an argument, and an argument can be won or lost by a choice on the page.
Audiences accept an argument they watched someone pay for. They resist an argument someone told them. That is the entire reason on-the-nose thematic dialogue fails: not because it is inelegant, but because it asks for agreement before the story has earned it.
The mechanism is the choice under pressure. Put your protagonist in a position where the easy option contradicts the theme and the costly option confirms it, then let them choose in front of us. Whatever they choose is what your film believes.
Counter-argument makes it stick. Give at least one character the strongest opposing case, played sympathetically and never mocked. A theme with no opposition reads as a lecture; a theme that survives real opposition reads as truth.