Try tipping the nose up a bit. Also, are you over reaching on the handlebars? Do you need to move the seat forward? You'll need to make sure that you don't compromise your knees if you move you seat. When your leg is at the 9:00 or 3:00 position (depending on which way you're reading your clock!) and you drop a plumb-bob from your kneecap, it should end up at the ball of your foot. If you move your seat up and find that the angle is too steep - in other words - the plumb-bob falls towards the front of your foot, you'll need to make the adjustment at the handlebars with a shorter stem. You never want to compromise your knees.

OTOH, the shape of your seat might not be correct for you (that would be the bicycle seat..har har har!). If it slopes down sharply from the back, it might be difficult to stay on the wide part.