Arguably the most important character in the Marvel Cinematic Universe is Tony Stark. Not only did his first solo film kick off the world's biggest film franchise, but he also serves as a great role model for overcoming mistakes and being stronger for it. Tony was always a self-made man, but that didn't make him infallible. And as the Infinity Saga drew to a close, Tony needed his fellow Avengers more than ever, and when they didn't show up, the universe paid the price.
When Tony stopped the Chitauri threat in 2012's The Avengers, he changed forever. He never quite understood the magnitude of the universe outside of Earth, and that lack of understanding manifested into fear, which evolved into PTSD. By Iron Man 3, Tony was scrambling to keep those around him safe from a threat that was still very far away. However, while the film would lead one to believe a kid helped him get past his issue, the reality was all he did was teach him how to focus his fears into something constructive.
From that point on, Tony's life went through a major makeover, and he took his moniker of "the mechanic" to heart. He began to figure out ways to fix the vulnerabilities of the world, starting in Avengers: Age of Ultron when he introduced sentient robots known as the Iron Legion. After finding a bit of success, Tony took it a step further by creating the peacekeeping AI called Ultron. This creation was just Tony's frantic attempt to keep the world safe, but when it backfired and tried to destroy humanity, he was ridiculed for it.
In reality, Ultron was just another attempt of Tony's to show his teammates he had massive anxiety about the world's threats. Unfortunately, none of them noticed the signs, not even Pepper. And as it turned out, Ultron would be the first crack in what would become a massive rift for the Avengers. Captain America: Civil War saw Tony push for the Sokovia Accords to ensure that there would always be heroes around to save the day. However, his trust was broken entirely due to the secret that Cap kept from him regarding the death of his parents, which left him alone in a world with nothing to fix.
When Thanos finally arrived in Avengers: Infinity War, it proved that Tony's fears were warranted. But by that point, there was no team left for him to tell that to. When Thanos ultimately succeeded in his plan, Tony lost nearly everything, so he accepted his defeat and began a quiet life as a family man. In truth, though, he felt abandoned by the people he cared most about because they weren't there at the moment of that defeat.
Being the self-made hero that he was, however, Tony took it upon himself to face his fears head-on in Endgame. Even though his friends should have supported his mental struggles, he was forced to do it alone. Now, make no mistake, the Avengers are heroes in every sense of the word, but they aren't perfect. When Tony said he needed Earth's Mightiest Heroes, they didn't assemble -- and because of that, the world paid the price.