It sounds like your saddle is bridging. The gullet width and angle of the bars of the saddle are the most important aspect of saddle fitting. Here are two articles that have information about saddles and saddle fitting.
Selecting a Saddle - http://www.extension.org/pages/29775/selecting-a-saddle
Saddle Fit for Horse and Rider - http://www.extension.org/pages/29771/saddle-fit-for-horse-and-rider
The following URL contains a gullet template you can use to determine the correct tree gullet to select in a saddle. http://www.horsesaddleshop.com/saddle-experts-recommendation.html. Once you accurately determine that, you should be able to select a saddle that correctly fits your horse.
