The way you find your happiness is by being happy with who you are.
A partner does not complete you, they complement you.
You are enough on your own.
If you aren't happy alone, I'd put money on you not being much happier in a relationship.
I know that probably sounds twee, but it's true.
I've been single for about 8 years now (I'm in my 40s) and while there are times that I miss having someone to share things with, I am genuinely also happy living my life.
I'm not quite in the same situation as you, since I do have kids, but again, while my kids obviously bring me happiness, they are not responsible for, or my sole source of, happiness.
I think it's really easy to think of all the 'what ifs' and wonder if life would have been better, but there's really no guarantee it would be!
I would recommend therapy (for everyone tbh cos therapy is great) to work through why you feel how you do and to maybe learn to appreciate yourself for just being you!