D.B.
Hi K.,
Wow, I can really sympathize with your position. I too had clinical depression (I noticed it post partum but later I realized, through therapy, that I had had it for some time). I took medication for a long time, did counseling, and now have been really stable since I changed my nutrition - that has helped a lot. We also do not have a lot of family close by, and I was 37 when I had my son, so I understand the age thing. He was also a difficult baby and it was sometimes hard to love him at first.
My husband and I made a decision that we would be a one-child family. The first thing I did was stop using terms like "only child" or "just one". We are a one child family, or we have a single child. The language is subtle but really key. Do not speak of this as if you are depriving anyone (yourself, your son) of other children. There is no one perfect size for a family. I have friends who have no children, and they consider themselves "child-free" rather than "childless". It is not up to others to decide what it right for you.
You are already exposed to plenty of societal pressure to give your son the gift of a sibling. You will hear plenty of things about how children need a buddy, someone to play with, someone to learn to share with.
Here's the other side of it: Our son developed great social skills because he spent time with adults, and also because he made friends everywhere he went. He had buddies in the neighborhood, and he developed great relationships with their parents. When we went on vacation, he went up to other kids on the beach or at the park or in a museum, and he made conversation. He always had a crowd around him on the beach as he developed elaborate systems of trenches and sand castles, caught little crabs or fish, or looked for a frisbee partner. He is incredibly creative, and his early love of building innovative structures out of Legos and K'nex has led him to pursue a career in engineering. He joined soccer leagues and ran track in high school. We did NOT overcompensate by signing him up for a zillion activities - he had down time and he played with other kids. He learned to share thru play dates and preschool.
We had the opportunity and the finances to go to museums, movies (still something we enjoy, and he's 20!), vacations, etc. We also took nature hikes and did "free stuff". We invited friends for Thanksgiving, others who did not have family around them either. We created a family, a community.
He has plenty of people to hold on to, lots of memories, and the confidence to make his way in the world. He developed into a real leader. He has heard stories of family members who are far away or who died before he was born, and he tells those stories to others as if he actually experienced them. You can enrich his life in that way, sharing cultural experiences and language (if your family speaks something other than English - not sure where you are from) and traditions and food from your family in Europe.
You can do this! It takes an adjustment in your mindset, and some confidence. See the benefits of any path that you choose, and make no apologies!
Good luck - feel free to write me back if you need to.