Suggest Remedy To Relive Constipation

Dear doctor, I am a 74-year-old female, 172cm tall, weighing 67kg. Two years ago I was diagnosed with colon cancer, had major surgery twice and spent a total of 3 months in hospital. In Sept. 2010 I had abdominal hernia repair. I have been constipated ever since the second operation. This was kept under control by using various kinds of over-the-counter laxatives. In the last few months the condition has become much worse, impacted faeces most likely. Strong laxatives and Movicol (macrogol) no longer work. My GP recommended daily consumption of oat bran soaked in milk and mixed with psyllium husk. This worked for almost 3 weeks, but now I can only have bowel movements by taking sodium picosulfate powder, which results in watery stools. - I would be most grateful for any suggestion. Many thanks!