I came across this quote that I found most helpful
Before you say you are not qualified to be used by God please remember this
Abraham was too old
Isaac was a day dreamer
Jacob was deceitful and a liar
Leah was ugly
Joseph was abused
Moses was a murderer and couldn't talk
Gideon was afraid
Samson had long hair and was afraid
Rahab was a prostitute
Jeremiah and Timothy were too young
Elijah was suicidal
Isaiah preached naked (I wonder what you would think if on Sunday morning your pastor preached naked)
Jonah ran away from God's calling
Job went bankrupt
Peter denied Christ publically
Martha worried far too much
Mary Magdalene was demon possessed
Zaccheus was too small
Paul was a murderer
Timothy had an ulcer and
Lazarus was dead
God doesn't call the qualified. He qualifies those whom he has called
(This Quote from Jim Daly in Focus on the Family)