It does get better op, but totally "get" where you are coming from - the sadness never totally goes away. Dd is nearly fourteen now and what with miscarriages/gynae issues we weren't able to provide a sibling for her. I'm very sad about this, as I am very close to my siblings and wanted the same for her. For a long time I felt I wasn't a "proper" mother with just one child, but at the same time I had to give up a job I really loved as my dh was travelling constantly when dd was small, and I worked part time in a job that was rather tedious, so I didn't feel I was doing either well!
Our childless friends, with notable exceptions, fell by the wayside; some seemed almost hostile to everything baby related which was quite shocking in retrospect.
However, I still felt hugely blessed to have one child. Things improved hugely when dd attended a small, v supportive primary school and there (dd and us) met fantastic friends whom we have stayed close to ever since! One or two of them are one-child families, but the majority are not.
Things eventually fall in to place and one begins to appreciate the less frenetic pace and flexibility having an "only" can bring; one tip early on is to be prepared to have an open door policy with regard to play dates, entertain a lot, and don't always expect equal playdates/ hospitality in return from families with multiple siblings because they are sometimes too busy. Equally, dd fits in easily when they are not (onlys learn to be adaptable!) Lots of her friends like coming here because they find it quieter and one or two regularly join us on family holidays/ trips etc. I still feel bad that dd won't have siblings to rely on when we are gone, but close friends do fill the gap to some extent.