These are words that describe people who feel sad and alone.
The most common word for this islonely. Someone who is lonely is unhappy because they would rather be with other people.Lonelyis used in all contexts.
I've been lonely since my friends moved away.
In US English, you can also uselonesomeinstead of lonely.Lonesomeis more informal thanlonely.
Someone who isisolatedis unhappy because they do not see or talk to others often enough.
I wouldn't like living out in the country - I'd feel isolated from the rest of the world.
In literary contexts, the wordsforlornandlornrefer to someone who is alone and unhappy.Forlornis much more common thanlorn.
The characters, a flighty heiress and her forlorn suitor, are a delight.
He's a lone, lorn creature.
Desolateand, in more old-fashioned contexts,desolatedare more formal words to refer to a person who is very sad and feeling alone. Both words are used more often to describe places than people.
After the market crash, desolate financiers left the field.
We are desolated, Colonel, that the regiment is to leave.