I would try to get a colonoscopy. It can diagnose way more stuff than just cancer. Looking back at my experience, the moment that things shifted for me (I was a couple years older, but still 30's) was when the Dr. asked if I would be willing to do a stool sample at home. I immediately said 'yes, please'. I had my colonoscopy within months. It could be sooo many things, blood tests can give a little insight, stool sample more, but a scope will give the Dr.'s as much info as is possible. Maybe offer to do a stool sample. They could catch C.Diff or several other things that cause stool issues. It'll also show that you are willing to take advanced (and a little unpleasant) action.
I'd also second the option of some documentation. You can just take a picture of what you eat (that will also capture the time stamp) and you can email or txt yourself when you are on the toilet. This can also be very helpful.
DX 11/09 RC Stage IIIb
12/09 Chemorad w/Folfox
Surgery 4/10 LAR, Removed Seminal Vesicles
Clean Margins, T3N2M0, 4/19 Nodes
6/10, 10/10, 1/11 Clean Scans, Normal CEA