Just to add to @sleepyfox
When you self learn, you hit obstructions in your path that you have to overcome before you can progress.
This can often lead to frustrating trial and error, and cargo culting, where you copy a solution that allows you to proceed, without understanding it. You can bake in a lot of bad habits at this stage, as you are simply unaware of alternatives.
Mentoring wipes out these roadblocks, and that is why it is so valuable.