In my opinion there is no rule book or definite answer on how to get your ex girlfriend. But you can get your ex girlfriend back by starting the communication. Ask her out for lunch or do something that she really liked when you two were dating. Don't frequent other girls, girls like to feel special and the only one in a guy's life, call her every now and then but not too often balance it all out and this will help get her back.
Honestly if you liked the girl that much you shouldnt of let her go, but the way that ive seen and experienced with guys is that they will tell you they still like you... and then ask you out later on! It doesnt work for me and im a 14 year old girl!! But it may work for other girls, so just try to tell her that you still like her then maybe ask her out later on! Hope i helped -Taylor