you lost the "easiest" weight on your own, and then some, before the surgery. if you hadn't done that, your initial post-op loss rate would be much higher.
but you did everything right and still losing almost 20lbs/month. you're right on track.
if you're struggling to stay under 900cal, i would suggest eating more solid proteins like meat. avoid soups as they clear quickly, leaving you hungry again soon after eating them.