Get to know your stirrups
While you are riding at the walk, you can kind of "pet" your stirrups, one at a time with the rythm of the horse. This sounds really wierd, but it's a way of just the tiniest bit focusing weight down into each stirrup, on the side that is moving forward with the horse's stride. So, at the walk, when the right shoulder moves forward, you press the tiniest bit more into that same side stirrup, then the other, then back and forth. IT's almsot like pedaling a bicylce, or "petting" your stirrups with the sole of your foot.
So, you kind of get to know them a bit.
Then, drop one, pick it up, drop it and pick it up. Reposition them on your feet, at walk and trot. Stand harder on it, then softer. Play with them a little. Get to know them.
Then, when you are cantering, feel for them again "you guys there?", then go back to feeling your butt on the saddle and your weight going down.
Remember, let the horse carry your weight, through those seat bones and downward. Let your weight down, your horse can carry you.