There are several candidates to “the biggest” lesson. As you are asking the one and only one, I would go with this:
Success in the corporate world has two faces, one is how much value you add to the organisation, and the other is how much successful you seem to other people in and out of the organisation. These two are usually not balanced, meaning that most of the time your image (lets call it face value) is either more or less successful than your added (or actual) value.
When your face value is higher than your actual value, it is easier to advance your position, increase your prestige, open some doors etc. It is also easy to make new enemies, attract more negative feedback from people. And due to increasing expectations, it is easy to fail.
When your actual value is is higher than your face value, it is easier to self motivate (the most important motivation type), to develop oneself without gathering so much attention, and usually people would not pressure you more than you can handle, maybe other than your immediate boss. But you would also frustrate, some people would not see you as “material” etc.
So, due to these characteristics, people tend to oscillate between these two situations. Now they are overvalued, then they are undervalued, and then, and then, it goes. It is important to remember, this situation is normal, both positions have positive and negative aspects, and should be acted upon accordingly. In fact most situations in life are in that way, with positive and negative aspects. Easiest time in your life is when you are in balance, not overvalued, nor undervalued. However it is also the most static time in your life…