They probably have something else planned for themselves. Either that or one just serve as a tool. But either way, it is better to move on than wear the chain of pain that serves no productive benefit.
Its sometimes because they are scared of the truth and stop themselves seeing the inner beauty, they hide, thinking they can escape it all. And basically life is cruel, you have to stay strong, even when you feel like diggin your own grave, crawling in it and drownin in your own tears. Love is complicated.
I doub they left you for hurting them, they probably used that as an excuse because they were scared and didn't want to admit it, they wanted to put the blame on you.
If that's the case, then you didn't need them. There are better people out there that will treat you like a princess and do everything they can for you no matter what. The reason you lost this person, is so you can find the one that is going to treat you well, you can't find them if you're with someone right? and to top it off, that someone isn't the person you're meant to be with. Your other could have meant a lot of you, and you could have felt like they were everything, but there is something even better out there. Don't think of your last relationship as a waste of time though, you alway learn something you can use later on, you're preparing yourself to be in the best relationship you'll ever get. Just keep your head up and stay positive. The minute you start to question yourself you'll start to question everything else thus making your world turn upside down, you don't need that, no one does. Keep a smile on.