If it makes you feel any better ljo28, I had one son and felt the same. I had my first son after not using birth control for ten years! Then another 7 years elapsed before I used fertility drugs etc and had two more sons. My first son was so lonely and I felt so bad all those years. But, be careful what you wish for!
Having more kids was absolutely extremely difficult for my original son to adjust to. I also had many complications and my middle son has disabilities. So, basically, my first son had to adjust to me sort of disappearing from his life and this has been just as upsetting as being an only child. Also, with such a large age gap, the kids are almost from different generations. (why I had the two together at the end).
And just yesterday, that longed for middle son said to me, "Wow, how lucky only children are, because all of the family resources basically get funneled into that one person"...
We all just have to try and count our blessings and understand that sometimes we are just along for the ride in this life.