The prayer for this week asks God that we might run without stumbling to obtain God's heavenly promises. That is a nice prayer, but stumbling has a lot more to do with me than it does with God.
Here's my theory; as Scripture tells us Jesus "He died for everyone so that those who receive his new life will no longer live for themselves. Instead, they will live for Christ, who died and was raised for them." (2 Cor 5:15 - NLT) So if he died for everyone, and grace comes first, than all have received the grace of God. All we have the option of doing then is to reject the grace (read stumble) and walk away from God's dream for our lives.
Those stumbles are usually related to things like pride, greed, lust, laziness, and plain old misunderstanding, and despite what the book of Job would have us say, God doesn't give us those things, we do them quite well on our own.
So we stumble and we trip and we fall, and God picks us up again and again and again because his grace is an ever-flowing stream. Despite all our trip-ups it is still possible to obtain what God has promised, so long as we always allow him to pick us back up, dust us off, and hand back to us the grace of his love.