Lonely (adj)

Unhappy because of feeling isolated from contact with other people

Solitary (noun)

One who lives alone, or in solitude; an anchoret, hermit or recluse.
 
What is the difference?
Loneliness is marked by a sense of isolation. Solitude, on the other hand, is a state of being alone without being lonely and can lead to self-awareness.
From the outside, solitude and loneliness seem very much alike. Both are distinguished by being alone. However, on the surface, all resemblances restrain.
Loneliness is a negative state of mind, characterized by an insulating image. One might honestly believe that something is missing. Loneliness is cruel, pain, a state of deprivation, a state of discontent characterized by a feeling of isolation, knowledge of unnecessary helplessness.
Solitude is the feeling of being isolated and not alone. It is a positive and constructive state of self-reliance. Solitude is ideal, a state of being alone where you are having a beautiful and acceptable business to yourself. Solitude is a period that can be used for some mental exercise, inner quest or development or relaxation. Deep reading includes solitude and the perception of nature’s beauty. Typically thought and imagination does similarity.
Loneliness is forced on you by others. Solitude is something you choose.
Solitude restores body and mind. Loneliness drains them.
 
We need to maintain a certain semblance of order, and some sense of steering our life’s boats. Otherwise, we get stressed, overreacting to slight annoyances and feel like we can’t keep up with something. For everyone, one of the best ways is to explore and enjoy, solitude.