It's interesting to read the account of Moses when God appears to him in the burning bush.  This is the most dynamic meeting recorded in the bible and yet it wasn't 'real' to Moses.  Moses comes up with excuse after excuse as to why he can't do what God's asking of him.  Take Noah, on the other hand.  That was a very small encounter between God and Noah that was recorded in the bible and yet it was VERY real to Noah.  God told Noah to build an ark and then God disappeared for 100 years and Noah built the ark while putting up with abuse, insult, condemnation, etc. from people around him!  Now THAT meeting was very real to Noah.

Moses's problem was that his sins from his past life were more real to him than all of God's powerful demonstrations.  His self-doubt was stronger and more real to him than what God was telling him.  His stumbling block was that he knew himself too well to think he could be used by God.

The same goes for us, I believe.  Our greatest stumbling block to serving God and fulfilling his call on our lives is what we know about ourselves.  God is not asking us to do superhuman things.  He's only asking us to do what is humanly possible and He will do the rest.  I need to remember to keep my eyes on God whenever I feel 'out of my league' or not capable of doing what He's asking of me.  That's all He asks.