I read this thread earlier and have been debating whether or not to reply. But I though what the hell!
The thing is, I don't feel like this. Now, admittedly dd is only 17 months, I do have a part-time job (working from home) and I was feeling lonely before I had her, but I can't help thinking that the being-in-the-background feeling you describe is not inevitable. Well, at least it's not inevitable that you feel like that all the time.
First, I think your family are being a bit thoughtless in their attitude. Could you mention it to them? (good idea to do this now, and you might get something other than baby clothes/toys for Christmas! ). If not, how about arranging an afternoon/lunch with some of your family when your ds is not there - "so we can really chat" or something. Obviously the latter option depends on how close you live and how often you see them.
It also sounds like some time alone with your dh would do you the world of good. In fact now you come to mention it, I could do with that too.
And friends too - how often do you go out without your ds? Just because you're a SAHM doesn't mean you have to be with him all the time.
Finally maybe you need something to focus on yourself - a course? a part-time job even (and that's not to say that not having a job isn't a valid choice, its just that having one can help sometimes). Or why not choose just one of those "other things you need to do" and keep doing it little by little.
Anyway sorry if this post is on the patronising side - its just I know you can miss some obvious remedies when you're low. Hope it passes.