There are a decent number of Twelvetrees cards about being lonely. Some have miniature poems on them, and others just flat out state the person is lonely! Below are some cards that I have.
“Me and the sky is blue ’cause we’re so far away from you.” Mailed 1934. Part of the message written on the back says “don’t take this card seriously.”
“You know I’m lonesome.” Mailed 1921.
“I ain’t lonesome ’cause I’M HERE, it’s because- YOU AIN’T.” Mailed 1921.