The biggest lesson I've learned since becoming a marketer is always be prepared for changes this business changes daily and to remember you're going to consistently work hard or you're going to consistently fail! What's been your biggest lesson so far?
Don't listen to everyone, don't listen to so called "gurus", just focus in a specific plan and work to achieve it. this is the biggest & hardest lesson you should be learn from your journey in the internet marketing industry.
Never give up. Usually a down time encourages me, personally, to work even harder... and once I am back on top again - things look way, way better then they did before.