It may have been long since your relationship ended. But he still thinks about you and feels a surge of regret for not stopping you when you walked away.
You may have moved on, and you may have had other relationships since you broke up. But no matter how long it's been, your ex will still remember you and miss you each time he thinks about the way you loved him. It was only after you both ended the relationship that he realizes how unconditional the love of a strong woman is. And he realizes what a huge mistake he made, especially when he's in these situations.
The way you made him feel, the love you poured into the relationship; he will never forget any of it. After breaking up with you, he could see that you were the only person who didn't hold his flaws against it. But instead, you embraced them and loved him for the person he was.
At the time, he may have put other things over you. He may have thought his career or his achievements were more important. But eventually, he understood that none of that mattered if he didn't have you by his side. He would do anything to take back the times he took you for granted and treat you the way you deserve to be treated.
It pains him to know that he has lost the love of his life, but what's worse is when he is constantly reminded of it by his friends and family. Even though he could see just how special you were, he never made you feel special then. And he now feels like an idiot for not stopping you when you chose to walk away.
He has tried everything to get his mind of how much he misses you. Eventually, he could see that his friends could give him the company he needs. But none of them can be the patient listener that you were. None of them can give him the genuine care and support that you gave.
It's not that he didn't try dating other women or getting into other relationships. But whenever he tried, he came across women who only cared about his status or his money. You were the only woman who cared about what's in his heart rather than what's in his bank account.
You weren't just his partner. You were the woman who lit him up with passion in a way that nobody else could. Even though he tries so hard, he still can't love a woman with the same depth he loved you. You brought out someone in him that he was proud of. But as you left, you took that side of him, too.
He misses having you in his life because you were someone who loved without any ulterior motives. But after interacting with others, he's grown tired of dealing with people who have hidden agendas. He wonders when he will meet another genuine soul like you.
The terrible experience of getting a taste of his own medicine made him stop taking relationships so casually. When somebody hurt him in the same way he hurt you, he deeply regrets having disrespected you, having been disloyal to you, and having failed to keep up his end of the relationship.
It may be months or even years since you both broke up. But he will always remember you as the one that got away. He knows that you have healed from the relationship and you're happy now, while he's still carrying on, missing you but doing everything to ignore it and live his life.