Answer:
First thought was Liberated.
Step-by-step explanation:
We know it's not overwhelmed or dejected as the poem depicts, "A poet could not but be gay" in other words a poet couldn't be anything other than happy. That leaves liberated and thoughtful.
I think liberated fits more because, "Which is the bliss of solitude; And then my heart with pleasure fills"
But thoughtful is also a good choice because, "What wealth the show to me had brought: For oft, when on my couch I lie.In vacant or in pensive mood
I feel like liberated would be more fitting because he's free from the loneliness