It's not just about squeezing your stomach muscles. If you just hold in your tummy the entire time without purpose (remember we train our muscles to develop muscle memory) then you're probably bracing ALL over, instead of being soft and moving with the horse.
If anything, it's about stopping motion in your hips for a stride or two (that is a half halt at it's simplest form)
It sounds as though your horse needs to work on their balance at the trot. Do they lean on your hands at all?