I interview badly but I am good at the actual job. Is there anything I can do about that?

Your read is basically correct, and it is worth knowing that it is not just consolation. Unstructured interviews are weak predictors of job performance — that is one of the better-established findings in personnel selection, and structured interviews with fixed questions and scoring do substantially better. Most final rounds are the weak kind. So the gap you are describing is a known property of the instrument, not an illusion.

That does not help you get hired, so: what the format actually rewards is a specific, learnable skill. Mostly it is producing a structured account of your own work under time pressure, in the first person, with at least one concrete number in it. Strong candidates are not the ones with better work; they are the ones whose answers have a shape — situation, what you specifically did, what changed, measured how. Vague feedback like "they went with someone stronger" very often means the other person's answers were legible and yours required the panel to reconstruct them.

Two things move it fastest. First, write out six stories in that shape and know them cold, so recall is not competing with composition while you are nervous. Second, watch your pronouns: people who work well on teams say "we" constantly, and a panel cannot score "we". Saying what you personally did is not arrogance, it is the thing being asked.

On nerves specifically — if the problem is that you know the answers and cannot access them in the room, practice under something resembling the real conditions rather than rehearsing in your head. Rehearsal builds recognition; a timed run builds retrieval, and retrieval is what fails. A scored practice loop gives you the feedback the real panel will not.