A colonic should NEVER be that painful! Some people do experience cramping, but it is similar to that experienced when one has diarrhea and passes rapidly as gas and waste are released during the colonic. Unfortunately...as with any profession...there are those who are very good at what they do and some that have no business doing it!...sounds like your discomfort was more a function of the therapist you were working with and their inability to communicate well with you.
Both the 'machine' and Woods gravity flow methods are considered closed systems...meaning that everything...inflow and outflow...are contained within tubing. A speculum is used in a closed system to which two tubes are connected...in an open system a narrow rectal tube is inserted and the waste is expelled around this tube into a well and drain that is directly underneath you. Personally I prefer the Woods method...both receiving and giving...it gives better results and encourages better communication between client and therapist....although it doesn't guarantee it...and unfortunately it is becoming harder to find therapists that use this method.
I always advise my clients not to eat for two hours before their colonic. This is primarily for their own comfort since abdominal massage is used during the session to facilitate releasing. Also, if a client is quite toxic they may become nauseas during a colonic as toxins will be stirred up and the body will want to eliminate them as rapidly as possible...eating shortly before the colonic can increase the nausea.
There is really nothing special that you should eat or not prior to or after a colonic...as long as you are maintaining a healthy Diet that includes sufficient fiber you should get good results...and also avoiding or reducing the foods that cause problems anyway....sugar, flour, dairy, processed foods, etc.. Being well hydrated can make a very significant difference to how well you release, though. Some people do use colonics as part of a fasting routine...juice, MC, apples, etc....but if you already know you are 'deficient' it is better that you focus your diet on replenishing with highly nutritious foods.
Frequency of colonics is a very individual thing...depends on how congested you are, digestive history, what you want to achieve, etc. Some will start colonics with a series where they have 1-3 colonics a week for a few weeks and then taper as their digestive functioning improves. Colonics alone are not sufficient to address digestive problems nor do they eliminate all of the mucoid plaque build-up...but they are a superb adjunct to a colon cleansing program and alterations to diet. You might want to try the psyllium and Bentonite cleanse during the time that you are getting colonics for more thorough cleansing...the colonics should enable you to do the p&b with much greater ease and results.
Colonics have been around for quite some time...and enemas had a very lengthy history prior to colonics. You are correct that a nutritious, high-fiber diet and healthy lifestyle with appropriate periods of cleansing (fasts, herbs, etc.) would make colonics unnecessary...and some of my clients only use them with maintenance cleansing or whenever they have been exposed to whatever bug is currently going around...colonics can sometimes knock it right out or excelerate the healing.