Many beginning writers submit material long before the manuscript is ready. (I know, I did the same thing.) It's the best they can do, and it looks all right to them, so they take a chance. They also glut the delivery system and interfere with editors and agents who prefer to devote their attention to reading polished, better-quality work.
I can't tell anyone when a manuscript is ready, but I can offer suggestions. Here's my first and most important one: Have your work professionally edited—at least until you get established. It will cost you money, but it's an investment in your dream. Go to a professional editor and not to an English teacher—unless you only want your grammar checked. Writing for publication sometimes violates strict rules. (And I'm a former English teacher.)
After the editing comes back, compare it with your original. Ask yourself why the editor changed your prose. A good editor makes your writing better; a wise writer appreciates the help.
Here's another tip: Never stop learning. I have two friends who were successful in the 1970s romance market. Today neither can get anything published. Neither kept improving. They sold books but they stopped growing.
Keep trying to find ways to improve. Read about writing. Take courses if you can; read blogs and books on writing. Analyze what you read. Ask yourself why one writer speaks to you and another one bores you.
One warning. You can find dozens of blogs and ezines on writing. In all candor, many of them are the work of not-very-good writers. Before you accept suggestions from a blog or ask a professional editor to tackle your material, look at the person's credentials.
A few months ago a woman emailed me that she didn't like the way I wrote and she was willing to become my editor. I try to stay open to someone helping me; however, she misspelled one word, had one run-on sentence of 43 words, and boasted that she had published five articles in an ezine. (Oh, yes, she was also the editor of that ezine.)
I didn't doubt her sincerity, but I wouldn't have trusted her editing.
Get professional help—that is, get PROFESSIONAL help.