There are a few bits of writing advice that I run across a lot that I think are bogus and may be damaging your confidence:
"You must write every day."
It's great if you do write every day, but there are plenty of successful writers who don't. Some have periods during which they research and periods during which they write. Some write a lot for a few days, then take a couple of days off. Whatever works best for you is right.
"You're not a real writer until your work has been published."
Was Charles Dickens a real writer the day before his first book was published? I'd say yes.
It may be fair to say you're not a professional writer until you've been paid for your writing, that's just how we define "professional."
If you're writing, you're a real writer. Anybody who doesn't think so just has a different definition of real.
"Your success as a writer is determined by how many books or scripts you've sold."
You get to set your own definition of success.
Is the best-selling writer automatically a better writer than someone who hasn't sold as many copies? Obviously not. The former may just be a better promoter, or have a more powerful agent, or a big publisher pushing his or her book.
My definition of success for myself as a scriptwriter was (and still is), "Am I making a living from this?" That's because I devote full-time to it.
The first three years I wasn't successful because I didn't make enough from writing to support myself. Ever since, I've been successful--although of course there are many, many scriptwriters who have made more money and had more projects produced. I respect and, in some cases, admire them, but I've never found it helpful to spend much time comparing myself to others.
Another definition of success is, "the things I'm writing these days are generally better than the things I was writing a couple of years ago--by my own standards."
"You're an imposter and will be found out."
This is one a lot of people tell themselves. Not only in the arts, although it's likely that it's more pervasive in the arts than in, say, accounting. The odds are that you're not an imposter and if you are, probably you won't be found out.
A belief of mine that may be more useful: "I'm not as good as I think I am on the days I think I'm really good, and not as bad as I think on the days I think I'm really bad."
A couple of useful questions:
When you find yourself worrying about issues like these, ask, "Says who?"
Literally, who says that?
What makes them the expert?
They may be an expert on what works for them, but does that makes them an expert on what will work for you?
Might the opposite be true?
If so, which one feels more constructive?
Try operating on the basis of that one.