I think loneliness is one of the worst mental problems to have. I'm not comparing to physical pain or disabilities or terminal illnesses here... just mental anguish that causes distress.
With many (most?) mental health issues you can feel isolated and alone, whether you are or not... but with loneliness you actually are alone. Even if technically you might not be actually alone in some cases, just disconnected, the point is you feel alone all the time. You have no one to confide in that listens, no one who can help, no one to bounce ideas off of, literally no one at all.
You suffer and you know everything is on you to resolve or make better... no one can help you, no one will help you... and while this is usually true for all things, when you are alone you are brutally aware of being alone.
Loneliness can, and usually does, cause all other things to breakdown. Even things you are good at and can manage otherwise, fall apart. Loneliness saps your energy, strength, willpower, it all goes right down the drain.