It's not just being alone. You can feel lonely even if you are surrounded by people. Loneliness means feeling isolated. It leaves you feeling sad or afraid. The older you get, the more likely it is that you will lose a friend or a relative.
New losses can bring up memories of old ones.
Grief support groups are excellent for people who have had these kinds of losses. You can learn what reactions are normal and how you can cope with the loneliness, while expressing your feelings with others that have shared your experience. Check with a local hospice to get information about grief groups in your area. Sometimes we need to talk to someone our own age to feel like someone understands us. Children and grandchildren can be great, but if you have been feeling lonely in spite of their attention, look for support from your own age group, too.