Yes, our minds are powerful things...but I don't think we can control disease as you're putting it. For those of us who are anxiety/depression/stressed prone...it would be almost impossible to require of ourselves to be able to control a disease.
Is it possible you're not able to differentiate stressful body symptoms with flare symptoms? Sometimes our bodies can be in extreme states with "symptoms", but we don't flare.
I should be flaring all the time..lol..but I 'm not.
But if extreme crisis hit...I'm sure I would be in a more susceptible state if I'm not using my meds.
You might also misunderstand "normal" people's dealings or control with stress...many don't show it, but pay a high price in other ways...sooner or later. Maybe they have a different way or organising or compartmentalising things. We're not privy to their entire lives regarding that.
I was in therapy for many years and my psychologist didn't believe I had depression. It's my constant companion, like a best friend I can joke with...but what I present to others isn't my depressed state. Anxious yes...even confident...never depressed.
But, he ws sick for a year and a half and I didn't continue therapy....and the depression came after a crisis in my life...when he returned, he saw it. By then, I was on Effexor, but at least I was able to function.
There are many ways we can help ourselves regarding stress...different classes of mindfulness, relaxation, meditation, therapy, tons of books, tons of things that can help get you onto a track of feeling at least in control. That may ultimately help with your stress and UC.
But...being in college, university or anything that has a high requirement of oneself...I doubt there is anyone who doesn't have any stress.
Add family life/relationships/friends/death of a parent or loved one .etc...everything in the game changes.
Hopefully, you can find your balance.