1. They give up too soon. As soon as the going gets tough, they fold. Often this is due to the obedience / approval seeking behavior taught within the school system. That may be fine for a student who just needs to graduate, but in the "real world", that is, post-school life, well it will harm you and your career often greatly.
2. Many folks label and then internalize valuable market feedback as "failure". Let me be clear here: It's only failure if you:
(a) label it as failure
(b) don't learn from it
(c) refuse to get back up
Stop labeling your experiences as a "failure" and start extracting value from these experiences by conducting post-mortem review. These reviews can be as simple as spending 10 minutes thinking what you learned, good and bad, from this experience, making some notes and seeing what could be tweaked or improved next time. Beyond that, see your "failure" for what it is -- a learning experience. And when we are learning, we make "mistakes" - they different between failure and mistakes couldn't be clearer.
(a) a failure is an outcome, not one event. It means you most likely haven't learned and the process is over because you didn't get back up.
(b) mistakes happen when we learn. It is an ongoing process and the most valuable part of the mistake is the lesson or lessons that we learn.
Start to reframe your view of the world that way and see how fast you will improve.