In my personal opinion…it’s a matter of personal opinion. Everyone’s line is a little different.

Most of the time I convince myself successfully that turning down social engagements is due to my introversion/genuine preference, or various situational factors (totally temporary and particular to the context…every time…)

I think it just takes time. To recognize your own patterns. And then, every now and again…there will be social engagements you’re actually interested in/excited about. And you’ll find yourself even avoiding these opportunities, too. Maybe you’ll tell yourself or ultimately convince yourself it wasn’t a good idea, or the mood wasn’t right…but deep down, you know it’s fear. Because you really had wanted to go.