I was widowed two and a half years ago, and I am sometimes lonely. I am sociable and busy and spend a lot of time out and doing things or seeing people, but there's still a lot of time - more than I would wish - when I am alone; sometimes this is OK but other times I feel lonely. As widowed people say, "I have plenty of people to do things with, but no one to do nothing with."
I think our generation may be the last in which it's quite normal to have been with one partner, as I had, for 50 years. We married young, and usually for life. Most younger people today spend some years as young, single adults, and may have a succession of serious relationships, or more than one marriage, perhaps with gaps in between, so being single is part of adult life for them. When my husband died, I had never lived on my own, never done a supermarket shop just for myself, never been in sole charge of a house, a car, a garden, etc, never had to come home in the evening to a dark and empty house, never slept alone for more than a few days at a time, never gone out without someone having a general idea of where I was going and when I'd be back ....
It has been a huge adjustment for me, and I know other widows will be nodding in agreement. The constant companionship of a long and happy marriage is something that can't be replaced, and the resulting emptiness causes feelings of loneliness at times. I think it's something I'll just have to live with.