First time ESA I got nothing (and, hilariously, I was too ill at the time to work out how to make a proper appeal
). First time DLA I was awarded nothing, appealed and was given lower rate care and mobility. Second time ESA was automatically put into the support group (cos of that student loophole) and didn't even have an interview. DLA renewal was supposed to go straight through, but got lost in the system and I'm still waiting for my money.
Adding in Rob's experiences and those of other people I've helped with it, my own experiences seem only too typical.
Have you thought through all your options and why you want to quit? If it's purely for health reasons, I strongly recommend talking through going part-time or taking a year or two off with your personal tutor. If it's for academic reasons, switching uni or course might be an option.
It's definitely not something to take lightly or to put the burden solely on yourself, so I'd reach out to as many people as you can (pastoral care at uni, friends, family and health-related people) and talk through all your options thoroughly first.