Yes. It is an awful feeling. It made it worse when she broke up with the dude after he cheated on her... Then they got back together a bit later. 😑 I didn't realize she had a boyfriend since she was pretty quiet about him all the time and he went to a different school.
Nevertheless, once I figured out, I began the process of moving on. I didn't want to waste time chasing something like this, and I also wanted to respect her wishes in it all. Feeing miserable is one way of putting it haha. I said "what the fuck am I doing with me life," stopped contact with the person for awhile, and went about my life, not looking back since.
Still on good terms with the person, but I basically said my peace, did a quick confession to her, and let the past be the past. 😅
Yeah, it really, really sucks. My advice is to find out if someone is taken as soon as you become even remotely interested in them ("your job seems really busy, how do you make time for your boyfriend/girlfriend?") Yes you risk missing out on people who wanted to break up for you but you'll spare yourself so much pain.
Yeah, it happened to me once and it sucks. The worst thing is that he kind of liked me back, even though he had a girlfriend. I didn't do anything though. Going after another girl's boyfriend is not something I would do...