In the Athrabeth
amdir is translated 'looking up'. Perhaps as the Athrabeth was written later than LotR Tolkien had developed the idea of two kinds of hope. He did seem to lack Amdir himself, especially later in his life.
Estel is a 'higher' form of hope, with a religious dimension that
amdir seems to lack. Of course, this would mean that Gilraen is saying she has lost all capacity for
estel herself - implying a loss of faith in Illuvatar.
Maybe she is saying that
estel is more necessary, & that the Dunedain need to remember that, & not put all their trust in themselves. How close this identifies Aragorn with a 'redeemer', or 'Christ' figure is another question - Shippey has pointed out the similarity in style & wording between the Eagle's song at Minas Tirith, & the Psalms in the King James bible - 'the realm of Sauron is ended forever, & the Black Gate is thrown down, & your King has passed through & he is victorious, & your king shall come again & he shall dwell among you all the days of your lives' etc (not a direct quote - don't have the book to hand). Which is not to get into the old 'allegory' argument, but Aragorn is definitely a kind of 'messiah' figure. Perhaps this is why
estel is more applicable to Aragorn than
amdir . But I'm really just speculating on all this at the moment, so please shoot me down if you don't agree
