I feel a little bit lonely & down now that most of my friends/family have settled down. Please advise?

I'm in my late 40s and have never settled down. The only reason I can think of is that I didn't meet a guy with whom I thought it would work with for life. I've had a pretty good job which enables me to travel, make friends all over the place and do what I like. I know that I look pretty, often get taken for 33 and am fit too. After my ex boyfriend of many years left me 6 years ago (he couldn't get his head around marriage), I've been more or less alone, except for the odd date. But I've never fallen for these guys and had just decided I like my life alone. However, I've just been at an old school friend's party last night and met with a whole load of old friends from 30 years ago! All with their teenage kids, doing great and for some strange reason I feel isolated and sad. Sure some are in happy marriages and some separated/alone. I'm confused as I always considered I had a great life. So why do I feel this way now when I know so well that many marriages fail and are charades? Please advise me?