Even in our own professions we benefit from skilled outside observation. Almost all professional licensees in any field are required to earn CEUs yearly to maintain credentials, even if they are recognized as a leader in their field.
Even if you are a professional firearms instructor (I'm not; not even close) get someone else to look at what you're doing. It's just smart.
There's been a lot of great feedback on this thread, there's a lot to be said about this. In my professional career I'm self-taught, but I spend at least 20 hours a week learning, researching, and "staying current." (Tech moves REAL fast, and if you don't do this, you'll become a dinosaur). That being said, I've never seen the benefit of certifications, or professional review. You either know it and can apply what you know, or don't. I've seen (in my industry) FAR too many people littered in certifications that didn't know jack squat about how to apply those skills effectively. "I'm an Eye-Tee Proh-Fesh-Uh-Nal" ... reminds me of the agent who shot himself in the leg in front of the school kids; a predictable outcome comes from ignorance.
This being said, you can't "solo" your way through martial arts of any form - you MUST have at least some form of peer review, outside observation, and feedback. There's always room for improvement. 18 years I've spent researching firearms, and have an encyclopedic knowledge of designs, specifications, strategies and tactics, and yet I still find myself learning more. Every single day I spend reading here and other sources, I learn more. Every time I go to the range, I learn something new.
There's multiple learning curves here - and on each, we're all on the same curve here - some at different points than others. I don't know diddly squat about black powder arms, very little about shotguns, etc. But I know quite a bit about other things. And in each of those other fields where I know more than most, there's people that know more than me. (This is true in my day job, too).
I'm not asking my wife to learn know how to detail strip a belt fed rifle or learn how to reload so she can start producing ammunition for the household down in the basement. Not asking her to learn to stalk game or study exterior ballistics to know what maximum point blank range is for a given projectile at a given velocity.
But I need her to be able to put a handgun bullet in an assailant, how to clear stoppages, how not to "freeze" if she's in trouble, and other useful skills; if something happens to me, or I'm not here to protect her and the kids. Robberies and other crimes are remarkably up - there's been too many bad things happening in this area lately. Even my cavalier son don't like going outside after dark anymore. There's been 15 heroin overdoses in this little rural county this year. Two houses nearby have been robbed SPECIFICALLY for their guns - by armed intruders. And so on, and so forth.
The world isn't getting any safer - if it were, I probably wouldn't even BE concerned - it was never an issue in the past.
But lately; I get nervous every time I leave. I'm afraid I'll come home from work one day and find my worst fears came true.