Lots of things can cause a positive FIT test. I believe only something like 1 in 10 are cancer and if you are younger, then the odds of cancer are probably even lower for you.

I don't think you are being unrealistic. Cancer is possible but it's probably not the most likely explanation. Could be something like Crohn's or ulcerative colitis or a polyp.

I didn't have a high FIT test, but did have a number of symptoms - rectal bleeding, weight loss, trapped wind and occasional (maybe once a week) loose stools and it turned out to be hemorrhoids, probably combined with the effects of my thyroid medication and the stress of the whole thing. 

Good luck with the colonoscopy. Hopefully it will turn out to be something minor. Let us know the result, if you are comfortable doing so.