Hi Annelife, I had exactely the same problem with my two girls. Don't worry about foremilk/hindmilk. To lower your supply let the baby finish one breast before switching to another one - even if it will take one day of feeding from one breast. This way your baby will get whole dosage of fat your body produced so far for given breast. Your body produces the perfect amount of fat for your baby. The sugar in the diet have no impact on the hindmilk supply. With my first baby I was eating mostly raw, a lot of fat and she had tummy troubles, too. With my second baby I was eating high carb and I didn't noticed any tummy troubles, only an oversupply and forceful letdown.
How to know when the breast is empty (or almost empty, because it is never really empty)? - When the baby's jaw drops down when she is swallowing. So, if the jaw drops with each sip - the breast is full. Progresively, she will take more sips between each drop. Breast is empty when the baby mostly sips to full her moth and rarely swallows. Then it is the time to swich breast, if she wish to continue.
See if anything in your diet can triger the oversupply. In my case it was the coconut water, watermelon (a lot of water), spirulina, fenel, dill, moringa and katuk (I live in the tropics). Kale is oversupply-safe.
You can try also herbs which helps dimminish the supply: mint, sage. The last one is very strong and can even stop the milk production if overdose - I never tried it. Mint is safer, I drank mint tea in desperate situations. Start with one cup and see if it does not upset baby's tummy - if you have no leaky gut problems she would be ok. Cubbage leaves didn't work for me. It is good for weaning not for diminishing.
To deal with forceful letdown be sure your feeding position is perfect, especially your baby's head position and if she feeds on whole aurreola arround the nipple. In other case she will extra stimulate the breast. Lie down position and keep the baby head alway above the nipple. Let the baby feed when asleep or just awake or falling to sleep - she will not stimulate you so much - somehow during the night it was always easier for me. See some videos on youtube and read LLL or find a lactation consultant which can correct the feeding position.
Your baby might be upset, but you have to be calm, positive, and smile a lot to your baby, sing, rock her during feedings (try a rocking chair, hammock, bouncing on a training ball...), anything what makes you and your baby calm. Eventually, it will get better and your baby will love your milk supply. Your feedings will be quick because she will learn how to quickly empty your both breasts.
I hope it helps:)
Also when you feel the forceful let down coming you can gently pull your breast off baby mouth and spray the milk to the towel. See if your baby is ok with that. And there is a technic of pressing the nipple to diminish the flow - look on google - I didn't like it.
Never use the pump - it will stimulate you even more. And wear a bra (I ussually don't).
You are welcome.:)