Sometime between abandoning Emma and his leg starting to turn into wood he much like the victims of the curse lost who he was, somewhere between 18-35 he didn’t do enough.
Maybe he had no way of finding her before now, so had given up trying? But when his leg started to give him trouble, he was drawn to the place they’d come through to our world, and in turn to nearby Storybrooke, where he found her, and everyone else. It was only then that he realised he could in fact still attempt to fullfill his promise and guide her into believing.
All I heard from his is he found an island to lose himself on and he never gave leaving a second thought to leaving till his leg hurt him. If he would have said to Emma “once I was old enough I started looking for you and it took me years to finally find you here, to finally get back to doing what I was suppose to all along…I’m sorry I never should have left you alone” then that’s something else! 😡