I had always pictured myself as the mother of many children, but after we had our first (and only), somewhere in my heart I knew that one was it for us. And one it has been for over fifteen years now! It was the exactly right decision for our family, just what God had planned for us (and He always knows what's best!).
My son is very content being an "only". From the beginning we were able to completely focus on him -- he learned to read at age 3 (not pushing him, he just wanted to), we've always been able to help him focus on his interests, we're all very close, etc. It's actually hard to put into words the benefits of having an only -- but it's certainly not a "bad" or "sad" thing. One kid can give you a whole lot of love! And we've been careful not to spoil him. He has limits, chores, responsibilities, etc.
My advice would be (if you do have only one child) to focus on the positive aspects of having one child. Make sure they have plenty of playdates (we weren't good at that), and make sure your child can play by themselves when you need them to. I really wasn't good at that, and ended up exhausted much of his pre-puberty life! Be sure to give them their space, especially when they get older. It's hard when you only have one kid to focus on. When they hit puberty and start pulling away, it hurts! There's no one else to give your attention to (except your husband -- not a bad thing!), but if your relationship has been good up to that point, it will be again after they've gotten through that spot. (He's gotten through the worst of it, and our relationship is doing fine, BTW.)
As for how he will fare later in life without siblings -- none of us know for sure if we'll still have our siblings when we go through difficult things, or if we'll be close to our siblings, if they'll even "be there" when we need them. My mother is an only child. She lost her father 25 years ago and her mother just this last year. Who did she lean on? First of all, God. Then her husband and her children. She's been fine without siblings all her life, so I promise you, if you have an only, the chances of their emotional survival later in life are quite good!
Anyway, what I'm trying to say is -- it's understandable to be sad if you've wanted more than one child and you can't have more. But being the parents of an only (and BEING an only) can have so many benefits and positive aspects that it's not the end of the world! As long as you move forward and don't dwell on what "could have been" or what you "didn't get", and instead put your energies into making your family a happy place, things will be just fine!