DC= Dear Children. DS= Dear Son. AC=?
I have two daughters, both independent and with partners of their own. We see them when we can. One lives in Norwich, so we only get to see them every few weeks, and the other's in Rochester. I'm not sure there is much difference between how we relate to sons or daughters with this issue, but then as I don't have sons I have no direct experience on which to draw.
I can say that I've had difficulties now and then, in the early days of DD2's first baby (our first grandchild), feeling pain because she didn't contact me more for support or allow me to help or simply be there. She has a disability and I could have done so much to help and support in those early days but my attempts were held at arm's length for the most part. I finally managed to adapt emotionally, boundaries were re-negotiated, and I realised that this was really more of a problem for me so I needed to work on it.
It's been said many times that we bring our children up to be independent and to live their own lives, hopefully with families of their own or in other ways of their choice. I think I'd worry if I was my daughters' 'best friend' but my (and DH's)relationship with both daughters is mutually supportive. Sometimes I feel 'needy' but the feeling passes. It's what my mother used to call 'gnawing through the umbilical cord' - actually a painful process, one which needs more time and work on acceptance, flexibility and adaptation than perhaps we anticipate. Accept your feelings, then - just 'notice' them, without acting on the urge to do something about them. 'Demanding' more from your son would be counterproductive. If you have the sort of relationship where you can talk freely about feelings, I'd recommend initiating that at some stage if you still feel emotional pain...just admitting to finding this stage of life more difficult to negotiate than you imagined, and that it will take time.
Don't be hard on yourself - your feelings are as they are, for now, but they will morph and change over time. 