Why is it SO hard to forget this one girl?

It'll suck but the best way to deal with this is to cut off contacts with the ex. Remaining in contact while the breakup is still so fresh to you will only make the break that much harder to swallow. There are other options to getting over a breakup then getting back with the ex (cause face it, sometimes getting back together is not an option). If you have a hard time meeting other chicks without thinking about her then stop meeting other chicks. Spend time with family and friends.Meeting a new person won't necessarily make getting over her any easier. Give your self a bit of time. Time can do a quite a bit of healing. Use that time to pick of a new hobby or something while you're at it.