I find it disturbing how much self-doubt there is floating around these forums, while there are FEW writers who appear to have any positive feelings about their progress and achievements.
There are a few things about self-doubt that we can learn from Master Yoda.
The story-killer self doubt is.
Infects us all it does.
Continue only will those who prevail.
To a certain extent, writers (and Jedi Masters), have to learn to have a bit of arrogance about themselves and their writing. You have to believe in yourself and your art first and foremost. Self-doubt will kill your muse, your imagination, your ability to plant BIC, and ultimately will show in your stories.
Ignore it. Be arrogant about your writing and your ability. Be overconfident. Just don;t be so arrogant and overconfident that you alienate editors and agents with a lot of egotistical claptrap about how you're the best writer since Stephen King and you're going to welcome the agent or editor onboard your bandwagon to fame and fortune. That ain't gonna' happen.
Sooner or later, that massive ego, that arrogance and self-confidence, is going to come up hard against the cold, hard realities of the publishing industry and the writing life.
It's a cold, hard, dirty business filled with potholes, pitfalls, and bumps in the road.
If you do not have confidence in yourself and your writing, who will?
You are your own cheering section and you must continue to be your own cheering section.
There are a thousand venues out there for your stories. Who cares if one editor turned you down? Send it out to the next one and the next one and the next one and the ones after that until that puppy sells somewhere, anywhere. No one's going to like everything you write, but someone out there will like something that you write.
You have to persevere and overcome. You must maintain your self-confidence and that requires maintaining an egotistical and arrogant attitude about your writing.
That is not, however, to say that you should not listen to editors and agents who ask for revisions. They know their markets and publishers. Assuming a compliant and cooperative attitude with them is usually in your own best interests.
But you have to sell yourself to an agent and editor very nearly as much as you have to sell yourself to a prospective employer in an interview. Every time you query you should consider the query letter to be your interview introduction. It is. You get your foot in the door and then you sell yourself via your self-confidence, your assurance, and your "can do" attitude.
Most employers appreciate a little arrogance and self-confidence in their interviewee's. It shows that the interviewee has at least been seasoned a little.
An agent or editor will appreciate self-confidence in a writer.
Just don't overdue it and tell them you're going to make both of you rich beyond the dreams of avarice. They know better - and you should too.