Diverticulitis

Dear Dr. My Mom of 81 years old has had three attacks of Diverticulitis in the past three years. She responds to antibiotic therapy . Ten years ago she had a Virtual Colonoscopy which showed up the Divertiulitis. The attacks seem to be seasonal- around this time- - why is that? It is a problem doing a Colonoscopy on her as she is a very tense person and they are unable to 'get into the colon' due to its shape- She refuses to have a Colonoscopy again- what is advisable at this stage? Thank You. I believe she should have roughage in her diet but this brings on diarrhoeal symptoms- please advise/