In "Birches", what does the speaker mean when he comments that he hopes that fate will not "half grant" what he wishes?

In the poem "Birches", Robert Frost describes a pastime he used to enjoy as a young boy.  He liked to climb high up in the birch trees, then, at just the right point, swing himself over so that the tree would bend, bringing him back safely to the ground.  It required a certain amount of skill to do that just right; he would have to climb "carefully with the same pains you use to fill a cup up to the brim, and even above the brim".  Timing and balance was everything; he would climb as high as he could, then swing over at just the right time so that the tree would bring him gently back down to earth, bending, but not breaking.

The poet remembers the feeling of exhilaration he used to feel, "conquer(ing)" the tree, climbing high to the top to escape the binds of the earth for a moment before returning.  Now that he is older, he longs to have that same feeling of escape when times are tough, when "one eye is weeping from a twig's having lashed across it open".  During these times he'd "like to get away from earth awhile...then come back to it and begin over".  He makes it clear however, that he wants the respite to be only temporary; he does not want "fate (to) willfully misunderstand (him) and half grant what (he) wishes and snatch (him) away, not to return".  Frost has no death wish.  He does not want fate to grant only part of his wish, the part about escaping the hardships of life, without making sure that it will grant the second part as well, the part about allowing him to come back down to earth.  He wants his whole wish, to be able to "climb...toward heaven", but not to stay there yet; he wants to make sure that the tree "dip(s) its top and set(s) (him) down again" when he is done.  The poet believes that despite the fact that it sometimes gets tiresome, "earth's the right place for love:  (he doesn't) know where it's likely to go better", and though at times he wants a break, in the final analysis, he wants to be sure he gets to stay on earth awhile longer.