My fast flow was linked with oversupply. I did multiple feeds from a single breast in a row to try to empty that breast completely before moving on to feeding from the other one for as many feeds as it took to empty it. In the mean time I'd express off just enough to keep the other breast vaguely comfortable. In hindsight I think a less extreme version of this would have worked better. But it did sort out my oversupply issues after a week or so and with that the fast flow as well.
I was also expressing a bit first and feeding in a upright semi reclined position.
I know it's hard but I found even when DS was hungry and upset it was better to take a minute to express a little first than to have him struggling with the really fast initial let down which would also make him frustrated and cause more issues later.

Good luck with it!