What is the mystery of the gospel? In reading through verses 5-6 it appears that the mystery of the gospel is that is is meant for everyone, not just a select group of people. Everyone is a member of His one body and shares the promise in Christ.
What is the power at work within us? if we have this power at work within us, why do we sometimes feel so weak? The power at work within us is the full power of God's Holy Spirit. I think we feel weak because we don't ask always ask for that power to be known, to be active, to be seen/felt. It could be that we rely on the shoulders up instead of our hearts.
God can do far more abundantly than we ask or think, according to the power at work within us. What do you want God to do? Would you consider it to be something that could only be accomplished by God's amazing and abundant power? And do you believe He is able to do it? I do believe God is able to do it. I have seen in first hand in some cases. I think for me I want God to change my heart and lead me to trust less in myself and more in Him. I want to do a better job of putting God first and modeling what that relationship means to my wife and my children. I have a feeling God can do this and will do this. I'm just not sure I'm going to be ready for it when he does.