Here are some fact sheets on fitting a saddle to the rider or the horse.
*Fitting the Western or English Saddle to the Horse
* Selecting a Saddle to Fit the Rider
To try to address your question, it sounds as if the seat may be too small. You would like to be able to put two to three fingers between your thigh and the fork of the saddle if it is a western saddle. You should also be able to lay your hand flat on the cantle behind your seat, and you should have a hand's width without the thumb from the top of the cantle to your seat. The pictures included with the fact sheets may be helpful. It may also be that your saddle does not fit the horse well, causing you to feel tipped forward. You want to make sure that the front of the saddle does not sit lower than the back of the saddle as this can cause you to pitch forward.
Be careful of wanting to get your legs too far forward as this usually causes the rider to ride on the cantle and stiffen up. This leads to bouncing, which is uncomfortable for the horse's back.
