Check out this sermon by Paige Benton Brown called "When Despair Invades Gospel Work: Learning from Elijah". It's also a great sermon just to listen to when dealing with your despair in general.
And at the end she quotes Joni Eareckson Tada:
"Jesus didn't pass me by. He didn't overlook me. He answered my prayer, and He said, "No," and I'm glad. A "No" answer has purged sin from my life. It has strengthened my commitment to Christ and forced me to depend on his Grace. It has bound me with other believers. It has produced discernment. It has disciplined my mind and taught me to spend my time wisely. It has stretched my hope, increased my faith, and strengthened my character. Being in this wheelchair has meant knowing Christ better--feeling His strength every day...The Lord brought me here that I might thank Him for the wiser choice. The better answer. The richer path."
