Oh I hear you OP. I was single right into my mid 20s, never had a boyfriend that lasted more than 3-4 dates, (and only had 3 of them!) and I got to my mid 20s and all 5 of my cousins, (4 female) were married, (all with kids after the first year or two.) Many of my friends were too.
I felt like there was something wrong with me, because no-one wanted to be with me/have me as their girlfriend. Then I met a man (my DH) in my mid 20s and got married after a couple of years, and quite honestly I love being married. I wouldn't want to ever be single again. We have been married over 30 years with 2 adult DC, and have had some bad times and good times, some average times and some amazing times. We have had a couple of times where we nearly broke up. It's been a wild ride! But for the past 10-12 years it's been great (as we've got older and don't really fight or argue now.)
Sorry, I know you didn't want the 'happily marrieds' on here, but I just want to say I GET how you feel and how lonely it feels and how you wonder wft is wrong with you? (Nothing by the way!!!) And the feeling of isolation and loneliness when you see other happy couples out together. As has been said though, things are not always what they seem, and whilst some couples present as happy, they are not necessarily having the best time! I have seen many a couple all over Facebook gushing over one another, and full of 'hunz,' and 'I wubz U,' and 'having the best life with this one!' And within 2 years they're divorced.
People who are very happy with their partner, and in love don't need to shout it all over Facebook. But you're right, whilst some couples are not as happy as they are presenting, some couples are pretty much happy most of the time. DH and I are. I would rather be in good/happy relationship than be single. But I would rather be single than in a shitty, toxic relationship.
You have children though @applejuiceisthebest so you were in a relationship once yes? What happened there?
People have given advice on how to get out more/join hobby groups etc, and try to make more friends and meet more people, but you say you've already done all that. It's hard to know what else to suggest sorry. I hope you meet your person soon. 