How can I slightly reshape my toilet's old-ish wooden door, back to the right shape?

You could try to adjust the hinges or plane off some of the edge of the door… :-/

Can you plane the door? That’s what comes to mind as your best bet, rather than trying to force it…

Scooby types faster than me, apparently. haha.

Is this something that just started happening? If not, I would plane it as the above have mentioned. If so, it could be that the wood is swelling due to the change in weather and will go back to normal eventually. If it is due to swelling and you plane it, then the door will be too short to close against the edge of the door and you’ll have that problem.

if it is wood, plane the top then paint to seal. In winter doors do not swell, they shrink. Bathroom doors may gradually swell because exposed to more moisture, especially if the top edge is not painted. Be sure top edge is painted.
If aluminum or other frame, solution is more difficult.

That’s a very impressive diagram
Make sure the hinges are Secure.

Wet wood expands. Dry wood shrinks, but not much.
Same goes for wood, oh wait that’s the same word.