ABABCB
The ending rhymes are:
ago
sea
know
Lee
thought
me
You assign the letter A to the word at the end of the first line, and then each different rhyme moves to the next letter of the alphabet. You assign the same letter to lines that rhyme.
Line 1, "ago" gets an A and since Line 3 "know" rhymes with that, it also gets an A.
Line 2 "sea" does not rhyme with line 1 so it gets a B. The following lines 4 and 6 "Lee" and "me" rhyme with line 2 so they also get Bs.
Line 5 "thought" does not rhyme with anything in the lines above it, so that is why it gets a C.