To take writing as an example, some people jump ship at the first sign of difficulty. The plot isn't gelling, the characters went AWOL, or you can't find the information you need for background research.
Others get through the writing part and give up either because some agents turn it down or because they can't figure out self-publishing details.
Whatever your source of discouragement, you will hear in the background the words, "It's just too hard." You may also hear, "It isn't fair," in which case, check out the post on resentment.
The more you repeat the unmagic phrase, "It's just too hard," the harder it will seem. Imagine that each repetition is like placing a rock in your way. Your goal is on the other side. If you say the phrase 10 times a day, that's 10 rocks. Uncontrolled repetition leads to building a wall.
Try to eliminate that phrase.
Replace it with others, such as "Maybe I can ask someone." "Maybe I can get a critique." "I might be able to find a helpful book or information online."
Remember the little train that could. Even if you're not sure you can, say, "I think I can."