First of all, it takes time for it to seem natural, so be patient.
If you are losing your stirrup, you are likely gripping up with your calves. Very common.
When the horse canters it's a 3 beat rythm. Count it as you go along such that the 3 beat is when the leading leg hits down. At that point your horse is most "downhill" in his position and you need to be most sitting back (as if you were riding downhill) in the saddle. However, you also need to be down into your stirrupss and have your seat bone on the saddle (unless you are riding in halfseat or two point). But in any case, as you count the 1, 2, 3 pause (horse has moment of suspension ) 1, 2, 3, replace the "3" with "down" and really think about your pelvis following the saddle downward, and your feet going downward PAST the stirrups until your feet drag on the ground . That mental image may help you stay with the hrose on that downward phase. During the upward phase, as long as you stay relaxed, the horse will carry you up. Let him do so.