First of all get the saddle in the correct height and fore and aft. Make certain that the Sit Bones are on the wings of the saddle and then adjust the tilt of the seat so that underneath is just being supported. Then check the ride- May find you are slipping forward so adjust up just a fraction till you stop slipping.
All of this is completely wrong if the saddle doesn't fit the width- is a brooks saddle that is set up differently or is a Bontrager saddle or you cannot fine tune the saddle-----Sounds like you may not have a brooks saddle so back to the LBS and get them to sort out the post at least.
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How long was I in the army? Five foot seven.
Spike Milligan