The world has learned that there is simply no point in doing a deal with Donald Trump because he will completely ignore it after he changes his mind.

I thought they did because Trump didn't like the agreement that he got in his first shitty term as president.

Either on purpose or by proxy

The trust of the world is dwindling...if not, already lost.

why do his supporters trust him to negotiate other trade deals or a deal with Iran?

Regular folks don't make money on futures contracts bought on ridiculous margins.

Just screw ‘em.

Most glaringly with Iran. Trump frames everything as being undecided, but also unimportant. He denies fixed goals so that his failures are easier to defend. The US relies heavily on free trade with its neighbours, but Trump can’t say that, he must pretend that he is unbothered , so that nobody uneducated will be aware of the significance when he screws it all up.
The world has not stood still either. If the trade agreement is renewed and things return to "normal" the US will find that things have changed. Whether it's lumber, or potash or aluminum, the volumes the US used to rely on are no longer available, Canada has made deals and signed contracts to export to other buyers. And the US auto-industry will return to a Canada where the former agreement not to import Chinese EV's no longer exists. They are already arriving.
Similarly with NATO, Trumps feigned disinterest has seen many member countries abandoning the US as the only supplier of arms. These changes will stay in place. Even when Trump ends up signing an agreement to return to things as they were, he has degraded and devalued them