So I dated a girl last year and I really liked her. We went our separate ways because we were both really busy. I still loved her at the end and still do now actually. Only problem is she's dating my best friend. But I am deeply in love with her, what do I do?

Time is sensitive at this point. If you really love her, don't let her get the chance to develop feelings for your best friend. Be that Alpha male, get aggressive, and win her back. NOW!

You had your chance, and now she likes someone else. Stay out out their way.

I'd stay away. You don't go into too much detail about how deeply you two were involved, but the fact that she went from dating you to your best friend is a fairly big warning flag. Without knowing any further details about that it's hard to give advice. It seems like she has moved on, but if you feel the potential rejection hurt is worth it, if you truly love her you can always tell her your feelings. You will risk losing her friendship and alienating yourself from your friend though.

First of all... it makes me wonder about the strength of that love you felt for her to be able to walk away just because you were busy.

Second... could it be you are just now regretting it because you see them happy?

Third... why the hell would your best friend date someone you were so in love with? That's a bit insensitive. With this in mind... he doesn't sound like such a swell friend. If you are seriously certain... beyond a reasonable doubt that this girl is who you want... (and you won't let her go like that again... ) then go for it... what do you have to lose?