So was I once myself a swinger of birches. And so I dream of going back to be.
Here we have another transition. The speaker shifts gears from a young boy he imagines swinging on a birch tree, to himself as an older man.
He seems to reflect on how he isn't young anymore.
Apparently the speaker can imagine this boy swinging trees in such great detail because he was once that little boy.
He wishes he were out there swinging trees like he was a boy again.
So all these details could be memories from his boyhood: conquering nature, girls sunning themselves, time alone to think about the natural world.
It's when I'm dreary of considerations, And life is too much like a pathless wood Where your face burns and tickles with the cobwebs Broken across it, and one eye is weeping From a twig's having lashed across it open.
The speaker wishes he could be a boy again when he's "dreary of considerations."
"Considerations" could mean thoughtful decision making – an important adult activity.
That's probably not what he's weary of, however. Instead, "considerations" might refer to the give and take of life. Older people have to give up things or pay for things that kids don't. This might be a way for the speaker to lament the fact that his life is now filled with responsibilities. What else might "considerations" mean?
Next the speaker compares life to "a pathless wood," meaning it's easy to get lost when there are no directions provided.
Lines 45-47 give the details of what happens when you walk through a pathless wood. You get sharp branches and spider webs in your face. These are all metaphors for the slings and arrows of life.