Hi B.,
We have an only child (a 5yo boy). The best advice I ever received was to ask myself, "How would I handle this situation if I had six kids?" I also spoke to a friend who is an only child herself and who is completely grounded and just a wonderful person about my concerns (btw, I know several adults who were only children, and I never would have known based on their personalities or behavior). She said that, often times, any issue an only child has gets blamed on the child being an only child. The same behavior in a child with siblings would be attributed to something else. She also said that she grew up around a lot of cousins and had to learn to share, etc., at an early age. We sent our son to preschool at a little over 2yo, which I think really helped in many areas such as taking turns, not being the center of the universe, etc. About having just one kid (because I have health issues): I like to think that we hit the jackpot the first time :) I feel really guilty sometimes when my son has asked us to give him a brother or sister, but these requests seem to come in brief phases. I hope some of this helps!
K.