Man, this is a hard, hard feeling to deal with. Believe me I know.
Here is my experience, and what I've felt.
I've encountered so many people, who just grab me with both hands with their intelligence, their looks, their personality, their integrity. I meet them and feel instant infatuation and excitement over the possibilities...until I see that wedding band, or learn she's engaged or in a long term committed relationship.
It really is a devastating realization. Because in an instant, a possible future has been killed in utero. For a brief moment there seemed grand new possibilities, and all that is swept away as I know I have no chance, that she has chosen another.
And it raises all kinds of questions. I start to compare myself to the guy she's dating or married to, and wonder, "What does he have that I don't?" Is he smarter, stronger, better looking, wealthier, more successful, more charming, more humorous, more adept sexually. More, more more, it's all about quantitative comparisons in which I come out as deficient and lacking.
And it brings up all kinds of fears for the future. I wonder if there are any great people left, or are they all taken? If I was going to find someone wouldn't it have happened by now? Why am I still alone, while everyone else seems to be finding someone? Why can't there be someone beautiful and smart and ambitious who chooses me? What if the fault is entirely our own, and there IS NO ONE?
As far as answers? I wish I had them. Because honestly I still remember every near miss and never was. I guess that's one of the curses of autism. We tend to form strong connections really quickly. I can remember everyone, and I still wonder what my life would be like if I had been what they wanted.
What I've tried to arrive at, beyond all my past negativity and loneliness and despair, is there are two options, which aren't mutually exclusive. The first is you acknowledge the possibility you will always be alone, and learn instead to love life as it is, and try to make it a rich one. The second is to keep trying. I won't lie, the odds are slim of you or me or any of us finding love, but if we give up, the odds are zero. So we have to take the slim chance, because it's all we've got.
I for one am still looking. I've not found anyone yet, and I doubt I ever will. I just don't think I have anything a woman would find desirable. But I'll keep looking in the hopes that I one day find a woman who does deem me worthy, and when that happens, I'll do everything I can to be the perfect mate. Until then I just keep at it, and try to enjoy life best I can.
I hope this helps in some way.