This is less about you being OK with being alone, and more about your GF wanting to have children. She basically asked you in a sneaky way of you want to have children, and to her you said no. So it is not about you it is about your future with your GF and she wants kids and you don't.

As for being alone I agree with you. In the US at least there is a stigma against loners, they are seen as losers, weirdos, someone perhaps dangerous, if you don't have 100 Facebook friends there is something fundamentally wrong with you. Most people rely on others for their happiness which is good and bad, because people can be unreliable or worse and shatter your happiness.

Happiness is great but for those with emotional issues like your anxiety, happiness can be a bit of reach. Again, there is that stigma in the US if you are not happy there is something wrong with you, or your lifestyle. This of course bullshit, and there is nothing wrong with contentment over happiness. For people with emotional problems extreme emotions like joy and sadness can easily get out of control. Good for you if you can throw 100% of yourself into joy or sadness but some people have to be more careful with their emotions, and contentedness is a great emotional state for mental stability.

In a way those who need others to feel good and ride the highs and lows of extreme emotion are in some ways addicted to other people and strong emotion. They simply cannot conceive of a life without it, and when they encounter someone like you OP they try to fix you, like a drunk pushing alcohol on a moderate drinker. For some, having to answer to dozens of friends and constant socializing is too stressful to maintain, but does not make them a loser or a misanthrope.