1. When you start learning you sound terrible... for a long time. It's very difficult to stay motivated when you can hear your problems.
2. It's very difficult to judge your own progress. I've started recording short videos during lessons which I can show to students weeks or months later when they're expressing frustration with their progress.
3. You have to enjoy practicing, since you have to pick up the instrument most days to maintain the dexterity, callouses, etc. It's very easy for a pushy parent to beat the joy out of practicing.
4. You have to understand the difference between playing that song you can play great, and practicing something you suck at so you improve. It's very easy to stagnate if you only play.
Really it boils down to, you're going to suck for a while, then you're going to think you suck even when you don't, and you have to keep enjoying the process even when you think you suck.
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