While they share the condition of existence of being alone, no two states could be more different. They are the Janus face of being an N of one.
Loneliness is a state of lack; a missing, an unfulfilled place of fear (even of death, and the idea that we “die alone”). It’s a feeling of utter despair.
Solitude, however, is something wholly different. It’s a state of fullness. Of being complete, at peace and in harmony. While not overtly connected to others in solitude, one is nonetheless connected — at one with nature, space, time and the universe.
And, interestingly, the choice is yours. You can change from one state to another in a heartbeat. It’s as easy as flipping a switch.