Aye Up,
Which 'Bergen' is it that you are using?
If it is the ex Brit mil 100/120 litre - is it a long or a short back?
If it is a long back swapping out the original 'waist' belt for a better quality one
might sort out the problem.
If it is a short back - does the shoulder strap attachment point to waist belt centre on the ruck match
your hip-to-nape of the neck measurement? If it doesn't the 'waist' belt will ride high and your shoulders will naturally end up carrying most of the ruck weight. (You may also limit your intake of air as too high a waist belt may restrict your lungs ability to expand!). I've seen blokes flake on tabs for that very reason!
Most military rucks are designed to be carried
above waist belt mounted webbing sets and so -
a) are shorter
b) have no shoulder to waist torso adjustment
c) don't have a load carrying waist belt but rather a stability strap.
I've recently posted the alterations that I have made to an NI patrol ruck for exactly those reasons and it works brill - but involved quite a bit of work.
For my own Bergen short back I swapped out the original shoulder straps for those from a civilian market set and made my own padded extensions using sections cut from the Brit military foam sleeping mat and some aereated? nylon mesh. (Back in the day it used to be foam pipe lagging and gaffer tape on the older (nasty!) framed rucksacks. But I digress
)
But that only solved a part of the issue -
I also created a Bergen-width pouch to strap to the rear of a
separate waist belt (as identified by dean442) which sits beneath the Bergen (as intended by the mil designers) and that shares some of the Bergen load onto the hips. (Sorry - no phot yet).
Re modifying a ruck if any work is beyond you, there is a shop down Hereford way which could help (it might be the one Bishop has named but not Im sure - someone on here will most likely be able to identify it).
Hope this helps