I've said before how you have to be several different people to be a writer. Sensitive, determined, and rhino hide all in one. There's one person though that you don't ever want to be, EVER! and that's the pissy negative one.
It's beneficial to be humble. It sure helps a lot when those rejection slips come in. But realizing you can improve your work and obsessing over the thought that it just isn't going to ever hack it are two TOTALLY different things. There are a lot of people writing out there. Thousands of folks scribble down ideas and send them into editors hoping to have those ideas ensconced in the immortal realm of published material. and you know what? Its true that there are people out there who never will be published. But if you're serious about wanting it, and willing to work hard and to learn, then you most likely will get published.
And when you're sending out your stuff, praying for it to join that ascended realm, remember that the world of publishers and editors is a small one. taking a pissy attitude of "my work is going to be published my way and I'll break any rules I want to!" will make it ten times more likely that you wont be published. Read the writers guidelines my friends. Then FOLLOW the guidelines!
I know it can be frustrating to trim your story or expand it or only send it to one publisher at a time. Heck no one ever accused me of being a patient person. :-) But like it or not editors have to allocate their time and they aren't just looking at your work or my work. They have lots of other writers too. Like the old saying goes You'll catch more flys with honey than vinegar. So pass on the vinegar, please?