Everyone has regrets in life - what they studied, what relationships they had, having/not having kids, getting married, being disrespected/bullied, job and career choices ....
You're as much the popular kid at school, as the "feeling a misfit" in college. We all are at different times.
You sound like a nice person, just discouraged.
So break down your life into three separate areas and you.
WORK: What job are doing? Do you enjoy it? Can you see yourself doing it for the next 40 years? Do you need to make any radical changes? Can you transfer your skills?
SOCIAL: Do you like your friends? Do you like your acitivties and holidays? Are you enjoying your social times? Do you want to make new friends or have new/different relationships?
FAMILY: Sounds like you've got a great family, if you were calling your sister when miserable at college. And people with great family relationships usually have the foundations in place for starting their own great family.
So if you feel it comes down to you, that one or more of these areas is making you unhappy, then make a list.
If you want new friends, get involved with new activities that you might enjoy. but have never tried.
If you want new skills, then take an evening or weekend course and learn some - which will also stretch and develop you.
If you want a new adventure, travel somewhere different with a friend or participate in a overseas charity event or organisation - like studying wildlife or cycling in Africa or going on a tandem sky dive.
Trying a new activity or adventure always means you meet new people - and usually people more suited to you with similar interests. More similar than the random bunch you went to school with.
Just incidentally, I had my first child at 32, my last at 42 and got married forever at 38. No IVF. Life just happens sometimes.
You're only going to get your 30's once. I'd make the most of them. You're in the prime of your life.
The grass is not necessarily greener for the 6th form brigade. You might find when they hit 40 they regret always playing it safe. Never venturing further than their small community. And regretting all the opportunities and excitement and adventure that they missed out on.