That's quite a profound question. I was talking to someone-who-shall-remain-nameless. Let's call her Ms. Negativity. She was upset because someone had made a disparaging comment about her new home. Of course, in my blunt manner, I immediately asked, "What does it matter what they think? What do you think?"
She just kept sputtering about how the mutual friend had hurt her feelings, etc. She didn't get the point that I was trying to make: it doesn't matter what someone else says or what they think. The only thing that matters is what you say to yourself when the other person shuts up.
Ms. Negativity could have said: "Well, Horace just doesn't know what he's talking about." Or, "Horace is a jerk with the social skills of a jackass."
Instead, she said to herself: "Oh, my God. Horace is right."
What Do You Say
Sure, we all recognize that my friend is insecure or she'd have just ignored what Horace said. But, how many of us act just like Ms. Negativity when it comes to something we're trying to achieve?
You write a manuscript and submit it to an agent. The agent soundly rejects it. What do you say to yourself? "Oh, my God. She's right. It is crap." Or, "That's just one person's opinion. I'll send it to the next agent on my list and see what she/he says."
What You Learn
Maybe a particular manuscript will never get published. So what do you say to yourself then? I'm a loser. Just paint a big L on my forehead. Or, "Okay, for whatever reason this manuscript won't sell. But my time wasn't wasted in writing and marketing it. My writing improved. I learned more about the publishing business. I'll take what I've learned and write another story. That's how you succeed and be happy in this business.
Success and happiness are largely dependent on what we tell ourselves.