I can relate to what you are saying though not so much when my children left home but at this stage of my life.
When my children started high school I went back to work and had quite a responsible job.
Twelve months ago I resigned to be able to travel with dh in his job. It sounds all great and I am seeing a lot but I do find it a little empty at times. I have to really make myself do small courses, such as travel writing, blanket embroidery, mess about on my computer etc. etc. Sometimes I get quite down but then get quite cross with myself because I am in an extremely fortunate position. I guess I miss the structure in my life and the fact that no-one really cares what I do all day.
When I am at home, I live close to a home for the mentally disabled, I have approached them to do some volunteer work but of course I can only do that when I am not away.
I am very close to my children and the internet is just wonderful for staying in touch.
I guess I haven't really helped other than you are not alone