I was talking with a co-worker yesterday, a guy who has been teaching a portfolio class at a local art school for over 4 years. He was telling me he made 3 students cry last week. Now if you knew this instructor, you'd know that he always makes a student or two cry every quarter. He's that tough.
And so we were commenting on how the students just didn't know it yet --but would in a couple of years-- that being brought to the brink of tears is actually a good thing. However bad it feels at the time, you will be a tougher creative in the end. Having your ego bruised once in a while hopefully helps make your work better and your skin thicker. His intent is not to make you cry, it's to honestly tell you when your work sucks. And I'm sure hearing that pretty much sucks.
Instructors make it hard because working in advertising is hard. This instructor is very, very tough on mediocre work because mediocre work will NOT be tolerated at any agency. Get used to it. I will bet you a million dollars that your future ECD will be equally (if not more) tough on your creative work. And your future ECD probably won't be too impressed if you burst into tears about it.
Thickening your skin, letting a bit of air out of your ego, and ending up a tougher person are all good things. And I'm sure that you'll have a teacher or two to thank for it in a few years.