Sniff, sniff, boo-hoo... use the adjective forlorn to express loneliness and feeling left out.

When someone is forlorn it means that they not only feel miserable but simultaneously desolate because they also believe they are alone. Forlorn is a very old word in English, and in fact it comes from the Old English word forlesen and means "to lose completely." Synonyms for forlorn all have fairly sad meanings: disconsolate, pessimistic, despairing, despondent, abandoned, depressed, desperate.

Definitions of forlorn
  1. adjective
    marked by or showing hopelessness
    “the last forlorn attempt”
    “a forlorn cause”
    without hope because there seems to be no possibility of comfort or success
Word Family