4 years of lower left abdominal pain and constipation

I am a 28 year old male.

I have had a dull pain in my lower left abdomen accompanied with constipation on and off for probably the last 4 years.

I have told my doctor about it before, and he poked around and told me to create a high fiber diet.

The pain and constipation seem to reduce when I am eating healthy, high fiber diets combined with adequate exercise. It seems that the pain and constipation resumes almost immediately after fiber intake is reduced.

Sometimes I am forced to drink magnesium citrate to create a bowel movement.

No blood in stool, but maybe a small amount of mucus only after bad constipation. Sometimes the pain will be just under ribs on left side, but usually it's about even with my belly button.

I want to see a GI specialist ASAP and get this taken care of. I have done some online research and discovered that I have many symptoms that possibly indicate Diverticulitis, irritable bowel syndrome (although I never have diarrhea) or worse.

I have also read where, if untreated, Diverticulitis could lead to colon cancer.

It bothers me that I've let it go on for this long, and hope it isn't something life threatening.

Just a little scared that's all. Should I ask for a colonoscopy?

Are there other screenings I should ask for?