Multiple enimas and manual removal of the impacted feces is nessessary.Treatments for simple constipation includes petrolatum based laxatives, fiber laxatives, and high fiber diets.
My advice keep giving the enimas regularly and doing what your vet said. It should get easier as time goes by.
Also, there is a surgical procedure to remove the diseased portion of the large bowel, but this procedure is not risk free. Chronic diarrhea and incontinence are complications. On the other hand, if done by an skilled surgeon, some cats have benefitted greatly from the surgery and have not had these complications.