Learning by Action

Experience is invaluable, knowledge is irreplaceable, something something high brow comment.

I really find it fascinating that we as humans learn by doing. Even when we're not trying to, we're subconsciously taking in information on what works, what doesn't, and using that information to get better at whatever task we're trying to complete. (For the most part) When we actively look at what we're doing, take feedback, and apply that feedback is when things truly get special in my mind.
With writing, feedback is the most essential component to becoming a great writer. We rely on editors to tell us what we did wrong grammar-wise, we rely on test audiences to tell us what they liked or didn't like, and we rely on reviews to tell us what everyone else missed.
Certainly there is going to be some advice and feedback that is wrong, not suited for a writers style, or reviews from people who just didn't like it and wouldn't no matter what changed (unless it was a different novel), but ignoring feedback is the worst thing a budding author can do.
I personally take all feedback seriously and try to improve my skills so I don't repeat mistakes. The results are fairly clear, my second novel was leaps and bounds better (at least according to the majority).
We may be able to get better alone, relying on our own thoughts and feelings as to what sounds good, but that can only get you so far.
Fortunately, humans are social creatures, and we've learned to help others. If you're without anyone to be in your test audience, ask friends and family to critique whatever you're working on for you. (They may be a little biased, but it's better than nothing.) Beyond that, there are no doubt workshops that can probably help you build your craft with professionals, or others in your situation, who can give you the valuable feedback you need.
Never stop learning, never stop working hard, and never miss an opportunity to grow.