Call it what you will but without it you're not likely to see success. Is it a decision? An attitude? Or just a natural drive to dig your heels in? Whatever it is you will need it. Don't worry, if you aren't naturally bull-headed you can work to develop that. In your professional life, at least.
Why is this determination so important for a designer? While design may seem like a glorious profession to most, those of us that have done it for a while know the truth. Design may actually be one of the hardest professions around. Why?? Regardless of your success as a designer you will face rejection. In fact, the bigger you get and the more clients you have the more rejection you will be subject to professionally.
So the question here is when the water starts to boil will you jump out of the pot? Think about that for a moment. The more successful you become the harder things get. The more people you are wiring with. I've often geared that designing would be great if it wasn't for the clients. Sometimes you will feel that way. And that's ok. But what are you going to do about it?
There are some telltale signs from clients that will clue you in that rejection is coming. Statements like:
"Well, don't take this the wrong way . . . "
"Let me tell you what I envisioned. . . ."
"It just doesn't feel right to me"
The list could go on and on. Do you quit? Do you hang up? Not if you have theses qualities we've discussed.