Good friends should treat you with respect and 'ditching' is a miserable way to treat a friend. Take time out with your friend and be honest about your feelings and let them know you've had enough. Of course they will play dumb and wonder what you are talking about, but ask them how they would feel if you 'ditched' them! Be independent and when someone makes you sad 90% of the time it's time to find a new friend.
Dont worry hun, this happened to me, this is what i did. Talk to her and say that it is nice that she has made some new friends, but dont forget the old, and if shes a true friend then she'll spend time with you as well. Hope ive helped XxXx