Chronic loneliness is not a diagnosis. It is a term used to describe loneliness experienced over a long period. This can happen for many different reasons. It might be that you grew up as an only child with a parent who was away a lot for work or a parent who was chronically depressed.
Loneliness is something that happens relationally. As a child, you reached out for connection, but for different reasons, nobody was there or responded how you needed them to.
As an adult, this can lead to a lot of fear and confusion around relationships, how they work, and how ruptures are repaired. It can feel difficult to trust that a relationship can be trusted, especially an intimate relationship.
On the flip side, when there has been loneliness in childhood, I often find a deep appreciation of connection and value for it.
If you think you experienced loneliness and are struggling in relationships, talking to a certified therapist could be super helpful.