Tony hated it. All he had, torn away in an instant. No mansion, no money, no machines. He tried to build new ones, but it was like he couldn't remember how. He couldn't explain it to anyone else, either, because yes, he did still demonstrably have the skills to physically build things and the engineering knowledge to repair a car from memory, but the spark just wasn't there. He tried blaming it on not being able to afford the right parts, but Pepper just took that as more lashing out. He couldn't bring himself to tell her that one night when she'd been sleeping he'd found an online IQ test and given up on it halfway through. He could recognize how much trouble he was having trying to solve for answers he'd hardly have had to think about before. As shameful as he felt about the expected low score, it would be even worse if it turned out to be a high one instead. One more thing he couldn't point to as a reason for the impotence he felt.
He couldn't think his way to a solution to his and Pepper's problems either. After the divorce, his desperation turned to depression. He missed her, he missed the man he used to be, he missed the opportunities he'd had with the Avengers to do something of use for the good of the world, and he missed JARVIS so much it was like a physical pain.