I use the word loneliness here in the context of a pervasive internal experience rather than as a response to external events such as moving to a new city, isolation in old age and divorce, although there will be some crossover. It is important to differentiate loneliness from being alone or solitude which are healthy and positive experiences.
Many people, and there are many, who experience this pervasive feeling of loneliness normally describe other experiences such as numbness, being in a fog, being shut down, excessive daydreaming, floating, through life and if we use more formal psychotherapy language, dissociation and depersonalisation. All these words are often describing the same thing and most involve a sense of hiding away from people and finding relationships difficult.
There will be multiple causes involved in this and the behavioural manifestations often include avoidance of social/group settings, excessively listening to music, sleeping a lot, technology addiction, drinking, drug taking, promiscuity, self harm, OCD and a myriad of other self soothing avoidance strategies. Alternatively, some people may over-compensate as a way to protect themselves by over-working, appearing over-confident, even arrogant and aggressive.
Whichever of the above means you use to deal with your loneliness, at its heart it usually involves a process of not being yourself, avoiding painful emotions and an inability to accept and show your vulnerability. This can often be caused by shaming experiences in the past and shame is noted as the loneliest of all the emotions as it a feeling that we keep secret and never share. In therapy the emphasis is on learning to live with your vulnerability, putting your life into a more favourable context by understanding the forces that have shaped it, developing compassion towards yourself and lastly, developing a deep acceptance of who you are.