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)