Most Helpful Guy
The reason for the others telling that he'll regret it is more than likely because they think he didn't deserve you and all those things. But if he'll regret it or not, that is not something that I can tell you. He sounds like he made a big mistake letting you go, but maybe he just didn't like you anymore and then he'll not regret it. If it instead is that he just lost interest for a while he will more than likely regret it after a while. It's all up for the future to say.
Most Helpful Girl
You never know it's usually months/years later when a guy may regret it. Usually when he can not get another girl, or when he can't get laid, or when he realises he cared/loved you after all. Or when he gets dumped by a new conquest. Do you want to be 2nd choice or the girl he runs to when he can't get what he wants? When guys lose interest , it could mean him forming an interest elsewhere if so move on and be happy or let your heart heal. He left you he doesn't deserve the satisfaction of knowing your pining for him. Anyone who lets you go doesn't appreciate or value your worth.