Ebbie, "forgot" really sticks out because my brain wants to hear "forgotten."

You could put the "forgotten" in the third line of the chorus, and rhyme the second line with the last:
Once I really loved you,
dah, dah, dah, dah, dah
Why have I forgotten
dah, dah deedle, dah

Sometimes it makes it easier to just swap lines around when the rhyme is too difficult.