Constipation and Celery

I've been working on finding a natural remedy for rather severe, at
times, constipation which arose suddenly about 2 1/2 years ago, in my
late 50s. The thing that seems to work the best, so far, is 2 large
stalks of celery every day. My "pellets" are much looser and not
compacted into one large mass. I was wondering if anyone might know of
WHY this is happening with celery? The psyllium, oat bran, wheat bran,
and ground flax seed haven't had as nearly as much effect as the
celery, which seems disproportionate to it's fiber content.