What testing was done to diagnose colitis? Hopefully it was a colonoscopy where some biopsies were taken and examined by the lab. If that doc office has a registered dietitian you should take advantage of that to create a diet that works for you.
I am also wondering what testing you’ve had done for your sciatica? Through a lumbar MRI docs should get an idea of what nerve is impinged (pinched). A qualified Physical Therapist should be able to help with that. For me, I was able to push off back surgery for a couple years with the help of my PT, ice, proper stretching and exercises, etc….