That's a tough one. The point of everything is to enjoy yourself. And the only way to learn a difficult skill is to make mistakes. If you can't enjoy even those mistakes, the desire to continue can disappear.
When learning to draw as an artist, you do rapid gesture drawings of say 1 minute each on newsprint. You don't attach to them. If they come out badly, who cares, you just do another one in the next minute, then another, then another. Gradually and eventually something good comes out of it. And you find facility. And your drawing gets better until you surprise yourself -- it starts to become unconscious and innate as an ability.
But if you start out trying to draw a figure like Davinci did, buy the most expensive paper, and supplies and just go at it as the only shot you have, and it goes badly, as it will, that can stop you in your tracks, and discourage you from actually acquiring skills.
Why not try taking your shelf apart, and cutting it down into a smaller box with thinner but straight sides. You will find a use for it, but besides that, just think of it as a practice piece. It isn't important. What is important is learning.
And then after that, try again to make a shelf with some new lumber. You will have had more practice by then, and will be aware of the potential problems in planing sawing and making fits. And when finished with it you will satisfy yourself about the quality of what you do and your own abilities. And you will have a good shelf, too.
Taking care of your own enjoyment and pride in your work is much more important than any other aspect of acquiring a skill. Your mistakes are okay, just part of learning, and convertible into something else.
If you can maintain enjoyment in learning, everything else will follow.