I sought a theme and sought for it in vain,
I sought it daily for six weeks or so. (1.1-2)
Any writer will tell you that facing a blank page is pretty much the terror of all terrors. What to do? Well, starting out by declaring your failures is one way to get going. Then again, maybe not.
What can I but enumerate old themes, (2.1)
Notice the strange syntax of this line? It makes it seem like this line is half question, half statement. After all, he has tried for six weeks to come up with other plans of action besides going over all his old work; there really isn't anything else that he can think of doing. But this line suggests that he'd sure like to have other options.
Winter and summer till old age began
My circus animals were all on show, (1.5-6)
Why compare creative works like poems to a circus show? For one thing, it foregrounds the fact that Yeats now sees himself as a showman putting on entertainment for an audience. And we're guessing that that's not a very positive feeling. After all, circuses aren't considered high art. Maybe Yeats feels his poetry has been cheap or tawdry.