solitude
Loneliness, anything other than being alone...
Of course, you can feel lonely when you are alone, never or hardly get visitors. No visit, no phone call, mail or ticket.
But also in a marriage you can be lonely, feel as such. Also at a party or in another company. Well, maybe you did. You don't feel understood, seen or heard. A topical theme and at the same time, of all times.
For anyone who suffers from this, this poem:

Loneliness, tears, wet or dry
They walk down your cheeks or over your heart
Cold and chilly, it sits you high