I think a lot of us have "isolation anxiety" these days, myself included. I think we live much more sophisticated lives than ever before, with a wide variety of interests and lifestyles, our identities carefully constructed over the years and with many boundaries, but in a way the result of that is that many of us find it harder to relate and connect with others.
I lived in many places over my life, sometimes of my choice, sometimes due to circumstances, and the knock on effect has been that I lost a lot of friends, and family in the process. Now my family group is very small, and so is my group of friends.
Like others, I feel socially sometimes on the edges or margins. I don't feel right now that I have a strong, solid friendship group. I long for more social interactions, but also I realise now that I have children that I find socialising hard work. Even an evening out after I put the children to bed doesn't sound that appealing as I am usually tired.
My way to cope with these feelings is to think that as we grow older, and have children to care for, we might have to slow down on socialising and take it a bit easier. Also, if you moved around a lot, it's quite natural to find yourself among people who are very different from you, and the bonding will not be as strong and deep as with people with a more similar background or history.
I also prefer to relate to people on a one to one basis, and find group dynamics more stressful so try to avoid gangs.
There seems to be an underlying expectation these days that women have to have a close female clique in their lives for them to feel fully supported or validated. It seems that we are replacing the myth of "finding the right" man, with the myth of finding the all embracing, supportive group of females. Both myths lead to disappointment in the end.