Here are two definitions of “loneliness”; each of which distinguishes “loneliness” from “solitude.” One is from Jackie Black, the other from Jean Vanier.

Jackie Black: “Solitude connotes the joy of being alone; lonelinessconnotes the pain of being alone. To be alone doesn’t necessarily mean one is isolated. The first is only a matter of physical separation; the second is a matter of physical, but also spiritual and psychological separation.”

Jean Vanier: “The feeling of being isolated is an essentially human feeling. It does not simply signify the fact of being alone. Loneliness is different from solitude. We may choose solitude; we may be alone and happy, because we know that, in other respects, we belong to a family, a community, the universe, and to God. Loneliness is a feeling that we belong to nothing, that we are cut off from everything and everyone, and that we are of no value. It is to feel incapable of facing a hostile universe. When we feel isolated, separated from others, we feel guilty for everything and for nothing, guilty for existing, condemned before even having been judged. Why? By whom? We don’t have a clue, and this feeling is like a taste of death.»

We would do well to only use these two words in the senses outlined by Jackie Black and Jean Vanier. So does the current parlance. An Internet search yielded 19,000 hits for the expression “die of loneliness” and only  375 for the expression “die of solitude.” One might say of loneliness that it is a solitude from which we die.

1-https://www.selfgrowth.com/articles/Are_Solitude_And_Loneliness_Different.html
2-Accueillir son humanité, Presses de la Renaissance, 2007, p.54.