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You won't get all 8 glasses in post surgery.. you need to just sip sip sip all day long and get in as much as you can... Your pouch is teeny tiny and inflamed and needs to heal.. so don't drink so much it hurts.. just sip sip sip... Eventually you'll be able to increase the amount day to day or week by week till you're drinking all you're supposed to..
My doctor said getting my water in post op and staying hydrated was almost more important than getting the protein in, at least for the first two weeks..
It definitely gets easier. I felt the same way after surgery; I would get full off of drinking or from taking my vitamins. I would always wonder/think, "how am I supposed to get in any food when these little things keep filling me up?!?!" But every day & every week that goes by drinking your fluids will get easier (as will eating if you're having a problem with that at all). Definitely sip, sip, sip all throughout the day to try & get in as much fluid as you can (to ward of dehydration).