Besides what everyone has sad, the difficult pregnancies, labor, post-partum, sleepless nights, taking care of sick kids, dealing with issues that kids might have as they get older, whether emotional, medical, learning etc etc, and the fact that you can never take time off from being a mother, all of which makes motherhood much more difficult than college, the big difference is that mothering is Real Life. If you mess up an assignment, did poorly on a test, had a bad day in college- while it may be devastating at the time, in the long run it does not really matter. But when raising kids, everything counts big time, for generations to come.
This is coming from someone with advanced degrees who spent many years in college studying difficult subjects and is a mother.