Drawing/painting takes a lot of concerted effort.
If you have really never sat down for several hours and worked on a drawing, you immediately have several things going against you:
1. You know you won't make something particularly good the first time. Why spend several hours of concerted effort on something that won't immediately pay off?
2. You aren't an "artist". We tend to categorize people by their current skillset, and constrain ourselves to the skillsets we are comfortable with. I don't know of any good ways to teach the brain around this without simply "pushing through" it.
3. Your goal is to do it "right", but you are going to do it wrong. Thousands of times. Every stroke of a pencil is a wrong stroke. The instinct is to immediately erase.
The beauty of painting is that you can't erase. This is why I would recommend a person begin with painting rather than drawing, so that every action is additive.
4. You have to start with purpose. The purpose has to be good enough to enthrall you for several hours. If you start painting something that you don't find interesting, you know at some point, you will get bored, and you will never finish, so why not stop now?
This goes for anything creative. How can you learn to write software without software to write? How can you perform music without music to perform? How can you build something without something to build? Creation needs purpose, if nothing else.
> On the other hand, most children don't have any problem being creative. Somehow by the time we become adults we lose that, and then think we're permanently incapable.
This is one of the most important barriers for us to overcome.
We need to practice doing things wrong, and creating garbage. Unfortunately, we constantly reiterate the claim that garbage is bad, and that we should do everything perfect.