Well I think we have all made mess ups in relationships, I dont think its the fact that you can never earn trust back, I think it's about whether or not you both really want it to work. If one person wants it more than the other it's not only trust that is the issue it's the whole thing.
You win some, ya lose some. If it doesn't work out, you don't need to bash the other person, call them names, or any of that, just pick up what's left, think of the good times and try your best to move on. Even if you have been wronged from that person. It's all about how you handle it.
I personally have learned that even when someone show's there worst side, if you truely give them a chance they can show you the best side possible. But I dunno. Maybe thats just me. We all learn. It's just how you handle it in the end I guess. Good luck :-)