I agree with Riku. You should have taken a chance. Now you are too late. However, there is a strategy that i use and it’s actually quite useful for attracting others or in my case, girls. You must be basically all types. Strong, compassionate, nerdy, independent, not caring about negativity, intellectual. You have to be what she wants. Or just not care and don’t pay her any attention. A few girls at a dance I went to were all over me when I did this. I was the most active on the dance floor and like four groups lured me over to them. Two of them brought me over about five times and others I just jumped into. I slow danced with a few girls too. Use a heck of a lot of charisma. Flirt but not too much. Don’t care about what people think. Humor goes a long way too. Be sure to get to know your partner. DONT BE AFRAID TO TAKE A CHANCE. That is a big mistake a lot of people make. I honestly don’t care if she’s not interested. Normally my interest in her leaves the exact moment she turns me down. For some reason after that... I don’t know why... they feel like they made a mistake and eventually try for me. I just don’t pay attention to their emotions after that but after like three months, they come back and it’s usually too late. There’s a bunch of strategies to romance someone tbh.
Just, I dunno, forget about him? If he moved on then you should, too. Instead of trying to get that one guy back try to go out and find another guy that likes you. It's madami work trying to get that one person back than to go out and find a new guy to date.
I also have to agree with Riku. You can't play hard to get with someone and expect them to wait, especially someone else comes along. The best you can do now is see where their relationship goes. If they end up breaking up then take your chance. If they don't, then it's best to just ilipat on.