Sometimes the hardest thing to do is nothing. This tree seems to be the only thing you have tried to grow, and all your attention is on this one plant. If I were you I would let this get on with it now, and allow nature to take it's course. Your tree will eventually grow more leaves I daresay and fix itself without any more attention.
Why not create a garden bed, and grow something else instead? Maybe some nice architectural grasses to start off.
There is no point dwelling on what has already happened. Keep busy