You sound like you weren’t emotionally attached to Iron Man or you enjoy pain. Personally, I’ve dealt with too much loss to enjoy losing an emotional investment. 

The Mist (2007) ending is kinda fucked up. Also since nobody brought up the obvious answer yet I have to mention Titanic's ending also that door could have easily supported two people.

The Mist ending is really good for me, the message was about the lost of hope of the main character, because of that you can see the mother who wanted to go into the unknow to save her childrens in the military truck. She take the risk and can save them, the father never can do that.

I like horror movies with sad endings (like Alien 3) in find this more realistic.

Sorry for my bad english.

Star Wars: The Empire Strikes Back also comes to mind. 

Though famous for "The greatest plot twist in the history of cinema", this so-called strength has forced people to unironically argue that new audiences need to watch the franchise out of sequence in order to properly enjoy it.  By itself, that's a massive red flag, unintentionally highlighting the underlying problems in the ending.  Whatever power that moment once had, it's long since been diminished by 25 years of extra content (the the writers/rights holders have always insisted we watch in order) and entire generations using it as a meme.  Instead of being an otherwise powerful revelation, we are treated to information we already knew through some of the worst acting in Mark Hamill's entire career. 

It doesn't stand on its own merits anymore, and thanks to the content of the Clone Wars and Rebels deflating most of the movie's worldbuilding, the non-ending renders Empire as actually the weakest installment in the Skywalker Saga.  Strangely, the same cannot be said for the other movies.  Revenge of the Sith, A New Hope, Rogue One, Return of the Jedi, even Solo are improved by subsequent movies/shows/games contextualizing their events on screen.  Empire is alone in its suffering and again, that's probably due to its over-reliance on a plot twist instead of building up to a satisfying conclusion.  With everyone faffing around through the middle, it was clearly telegraphed that having an actual ending was an afterthought.

Worse, when it brought the lazy storytelling convention of "To Be Continued" to the big screen, a number of franchises were misguided enough to follow suit, damaging the structure of their own trilogies in the process.  There's a good reason almost no one argues that The Matrix Reloaded had a tight narrative with a satisfying conclusion.  No, by foisting the burden of a proper ending to the third installment of a trilogy, the writers were given leeway for unnecessary bloat and allowed them to write themselves into a corner.  If Empire can get extra points for raising expectations of a sequel, it sure as Hell loses points for this.

It is actually frustrating that neither the Empire, Rebels or main characters are closer to reaching their goals by the time the credits roll.  The only one who actually managed to push the story forward was Boba.  Unsurprisingly, that's what sets the stage for Return of the Jedi.

I don't know about worst movie endings but I have to say for most recent underwhelming movie endings I would have to say Escape Room 2.

I also loved the ending of The Mist, it hit me like a freight train but I absolutely adored (SPOILERS) the tragic yet morbidly poetic nature of it; that if only they'd held out hope a few more minutes, they would've been okay. 

Star Wars: The Rise of Skywalker

"There's been no one for so long. Who are you?"

"I am Rey"

"Rey who?"

"Rey Skywalker"

Bitch no you ain't

Sleepaway Camp is easily the worst I can remember. Don't look it up. You'll regret it if you do. It's that terrible.

Of recent films I've seen, Cloverfield Paradox, I think the last minute made everyone hate the film. You sort of have to watch the entire film to see why this ending is garbage. But it's not bad... then you see this "twist"

Honestly Cloverfield Paradox is complete shite even without that ending.

Its a pretty crap film in its own right but the way it uses the Cloverfield name to sucker in fans of the original only to do the worst job possible of connecting them is just insulting.

Give us a proper sequel to the 2008 film already, I am sick of all the misleading spinoffs.

Luckily, they recently announced that one IS in development. Can't wait, loved the first one. I consider it not only my favorite found footage film (sorry, Blair Witch Project, which I personally find overrated), but one of my favorite kaiju films. I must've watched it a dozen times on DVD and rebought it in 4K recently, which looks great BTW. The marketing was some of the most genius stuff ever conceived at the time.

Cloverfield Paradox and 10 Cloverfield Lane started out as original films before having the Cloverfield name slapped on them during (post?) production to sell more tickets (or Netflix streams in the case of the former).