"I found out a man I had developed deep feelings for was in one and had no intention of a relationship with me. Pretty selfish."
So you assumed he was single? Having been the third wheel several times, beginning when I was 19, I know how you feel, BUT in one instance I knew they were a couple and went for it, developed feelings for one of them, had a good sob, then moved on. I forgave myself for it because I was young and very inexperienced. It was 1974 and there was an assumption that if you saw someone out and about looking for a tryst they were single. Really bad assumption.
So it was really pretty simple. I started asking guys that were interested or showed some sign of reciprocal interest this, "Are you single?"
It sure made things a whole lot easier. If they lied it showed how shitty they were, and had zero to do with me being foolish or unlovable. Although my brain certainly tried to find fault with me, lol!