I knew that feeling once, eventually just forced myself to go righty as most games made sense like that anyway.
As for the bindings issue, coming from someone who's coded small XNA projects (the technology Terraria used) I can tell you that by default there is no lefty check in XNA. Terraria looks for left clicks or right clicks and has no care as to whether or not you're on lefty mode or not. I suppose a simple toggle in options or the ability to remap the mouse would work also though. But for now I'm sorry to say that there isn't a fix in game, it will have to come in an update if it ever does.