I hate to repeat myself, but lesson learned, I got it, office social structure is nothing like the school studio.
Yeah, the two are very much unalike. Additionally, the work environment of today is nothing like it was 20 or 30 years ago.
First, there is no more loyalty. A company will throw anyone out the door if it's for the benefit of the company. No matter how good a job you were doing, if your department get's outsourced, you're history. Years ago the company would see if they could retrain you for a different department, or help you find a job at a different company. A few companies even paid their employees to do community service work during a down time. No more. You are a commodity, highly replaceable, and a dozen like you sitting in the HR reception room.
The old idea was to find a good company and work your way up the ladder. The ladder is gone from most places. If there's a merger, you may find you were on the wrong ladder. Today, the ladder is gone, welcome to the jungle gym.
I once went to work for a company and two and a half years later there were only two people that had been there longer then me. I didn't need a brick wall to fall on me to tell me that no matter what kind of good job I did, I'd soon be gone. Before I could leave, one of the two with seniority was also gone.
Finally, full time with benefits is becoming a rarer thing. Furthermore, a company will decree a 10% cut in employees. If you make the cut and keep your job, you can expect a 10% increase in what they want out of you. Once you get up to speed on that higher workload, there comes another 10% cut, etc.
I went totally freelance a few years ago. I make less money, but I work less too. There is no stress, other then what I want to impose on myself, the working hours are great and when a nice day comes along, I don't have to ask anyone's opinion to go enjoy it.
One last thing. Write the email you want to send to your old employer, but don't send it. File it away and look at it in a year or two. Then you'll know why you were advised to not send it. Nothing good ever comes from spilling your guts or emptying your spleen. You'll see.