As a writer, it's critical to manage your emotional state of mind because it influences the quality of your writing.
If you are in a state of mind in which you imagine you'll hit a homerun for your client, chances are you'll write a winner. But if you're in a state of mind in which you worry that you'll fail, then chances are you'll write a dud.
To manage your state of mind you must first be aware of it. Secondly, if you find your state of mind is a hindrance to good writing, you must know how to change how you're feeling.
It's easy to let daily challenges negatively influence you: interruptions, deadlines, bills, taxes, a client's negative reaction. The trick is to recognize what's happening and self-correct. In other words, choose your response.
Sometimes when I'm feeling pressure and I notice my state of mind isn't the best, I'll go to the gym to exercise. I'll do this even if I'm under deadline. Why? Because the endorphin rush puts me in a positive state of mind. I will ultimately end up writing faster and writing better than if I'd chosen to continue writing without a break.
I've noticed that I can change my emotional state by watching certain kinds of film and TV. For instance, watching extreme snowboarding videos gets me fired up. I love to snowboard, and I love to see a guy catch big air and stick a landing.
Also, this year I decided to start following football. I had never done this before. I discovered I like football. When I see a guy score a touchdown on a hard drive, I get excited. More than once, I surprised myself by yelling "Yes!" and pumping my fist.
Seeing other people win is a great way to put yourself in a winning state of mind.
Other times I'll go out, take a walk, and get some sun... or take a few moments to read a motivational book. These things are uplifting to me, and are helpful when I'm feeling stressed out.
I suppose any number of things can put you in a positive "I can do this!" state of mind. You need to figure out what works for you.
The critical thing to remember is that your emotional state of mind influences the quality of your writing. If you actively manage how you feel when you write, you'll automatically improve your writing.