The simple solution is, if you've read it through, edited it (more than once) and read it through again on paper - always a sure-fire way of spotting a hidden horror you'd missed first and second time out far better than on screen - and got someone else to read it for an objective view, then you've done as much as you can.
Anything else is just fiddling and a waste of time and worry.
Throw off the doubts and send it off. Then get on with the next project.
You'll get a response sooner or later. In the meantime, rather than staring at the post box or checking your email every couple of minutes, push it away by doing something positive.
My New Author Profile in the same issue covers Peter Breakspear, who gained publication of his first book, 'End Point' by winning a writing competition run by WM in collaboration with Matador Books.
In addition, Peter got something most authors never get to see: to follow every stage of his book through design and production.
As he points out in his interview, winning a competition was the only difference when it came to producing a book. What he shared with all other writers is checking his facts and doing his research.
And most important - getting the story on paper.