How to pray when life is hard

We prayed for nearly a year for our missionary colleague who died just over two weeks ago, as he struggled with the ups and downs of cancer. Our prayers strayed wildly between two possible outcomes of this: that God would heal him or that God would take him home to heaven. Of course our hearts were more in the first prayer than the second, but we prayed them both nonetheless, knowing that obedience was important.

Only a week or so before our colleague passed away I was reading a portion of Timothy Keller's book, Prayer. I mustn't have been near my computer, because I typed all of the below quotes on my iPhone. I think they're worth sharing now, for while that particular situation is now resolved (we know the answer to our prayer) and now we pray for his family and the witness he left behind, while we live on this aching earth we're going to continue to encounter people dying. More and more as we age.

A quick aside about the witness our colleague left behind. There were two parts to his funeral. The public part was all in Japanese and there were something like 60 Japanese teenagers there. They were friends and soccer team-mates of our colleague's two younger sons. The gospel was clearly proclaimed. We continue to pray for these seeds that were sown in a way that never would have been possible if God had answered our prayer for our colleague's healing here on this earth.

Nevertheless, it is good to know how to pray in these situations. Keller sheds some light on the situation by looking at Augustine's teaching on prayer:

Even the most godly Christian can't be sure what to ask for when we are enduring difficulties and suffering. "Tribulations . . . may do us good . . . and yet because they are hard and painful . . . we pray . . . that they may be removed from us." Should we pray, then, for a change in circumstances or just for strength to endure them? Augustine points to Jesus' own prayer in Gethsemane, which was perfectly balanced between honest desire . . . and submission to God" p. 87, 88.

And further on, he makes some startling statements about prayer: 

Augustine . . . "argues that not only can we grow in prayer in spite of these difficulties but because of them. He concludes his letter [to a young widow who'd lost almost everything in the sacking of Rome in 410AD] by asking his friend, "Now what makes this work [of prayer] specially suitable to widows but their bereaved and desolate condition?" Should a widow not, he asked, "commit her widowhood, so to speak, to her God as her shield in continual and most fervent prayer?" What a remarkable statement. Her sufferings were her "shield"—they defended her from the illusion of self-sufficiency and blindness that harden the heart, and they opened the way for the rich, passionate prayer life that could bring peace in any circumstance. He calls her to embrace the situation and learn to pray" p 88.

Most of us aren't widows, but shouldn't we too turn our griefs, our sufferings, into a shield, one that helps us pray all the more fervently and protects us from going astray in our own strength?