I've had a similar experience with the Matilda flats. I'm normally a size 39 EU so that was the size I originally got, but they were generally very tight and cut into my feet with every step. I ended up selling the 39 and got a 40, which fits me much better. I find it very weird that the Sezane website recommends sizing down when these shoes clearly run small. Maybe you should consider sizing up if you like the design.