If you feel it when you aren't alone, could it be something other than loneliness? Are you grieving?

Sometimes I journal to get to the bottom of what I'm feeling. Maybe you already know why you feel that way. Just throwing out ideas here. I once felt desolate, like I was the last person alive. That was from suffering a heavy loss and betrayal.

Besides Sarah's great suggestions, you could try therapy, meds for depression, distraction, like researching some interesting subject online, going outside, walking for endorphins, taking a class and learning something new.