Though the speaker may be seething with frustration and even anger at God, he knows that he must tread very carefully if he wants to express himself. He has the skills and intelligence to do great things out in the world, but he has been tripped up by a seemingly trivial problem: his light has run out. Unfortunately, this unlucky event throws a big wrench in his plans to be useful. The speaker imagines that he could have become a famous politician or even, say, the author of one of the greatest epic poems in the English language. (Cough, cough). But now he can't possibly do any of that, right? (Cough, cough, cough.) He's a hard worker, and he can't just sit still! Can he?
You get the impression from reading the first section of the poem that the speaker just wants to shout, "You can't seriously expect me to do my Christian duty without vision. This is a joke, right?" Fortunately, the speaker has a little guy in his shirt pocket named "patience," capable of popping out and delivering a pep talk whenever it looks like the speaker is about to lose his cool. The important thing to remember is that "patience" belongs to the speaker.
Finally, we can see why the speaker thinks he would be so useful to God and to society: he is incredibly smart. With his ability to pack complicated arguments into a few brief lines, he would make an amazing lawyer. Also, he probably knows the Gospels inside and out because the "Parable of the Talents" is one of the least frequently quoted stories from the New Testament. On the other hand, the speaker still has "half [his] days" left to live, so maybe, just maybe, he'll still get around to doing great things.