The way it reads to me if you read the rest of the verse about fulfilling purpose is that its more of a plea from the author. ("Do not forsake the work of your hands"). Its sort of like how in the previous verses he says all the kings of the earth give thanks... which I doubt they do. My other theory is that this just isn't translated all that well from thousands of year old Hebrew to modern day English. I don't really know, but it is interesting.

I think I go through a similar breadth of emotion each day as this author. He starts the day out great, singing praise, giving thanks. Then reality hits like a rock and all of a sudden he's calling out. God answers and strengthens his soul and he can't hardly believe it so he sings some more praise. Then reality starts sinking in again and he looks back to what he was thinking earlier in the day, and convinces himself that God has a purpose for him and will deliver him from his trouble. Then the doubt sinks in and he's back to pleaing for God to not forsake him.