The thing is, to a certain extent the way the story is presented is the book, and every mistake that a reader (and that would include editors and agents) notices is a distraction, a bump in the road that throws them briefly (if only for micro-seconds) out of the story.
If your story is indeed brilliant with an enthralling plot, excellent pacing, and engaging characters, an editor would almost certainly be willing to overlook occasional minor flaws in grammar etc. -- assuming these things didn't put them off the project before they reached the part of your story that would draw them in so completely that they would stop noticing such small mistakes. But if your story is less than brilliant, if there are other flaws as well, that you can't see but an editor would -- then all this goes into the tally of reasons not to keep reading, reasons not to buy. They have so many other manuscripts on their desk, more than they could ever hope to read through in their entirety or with the kind of attention they might like to give, that they have to make fairly quick decisions on what is going to be worth the time it would take for a more thoughtful and careful reading.
Editors get paid to acquire manuscripts and get them ready for publication. The days when they had the time and the mandate to discover new talent and carefully nurture it along are long past. Time spent with one writer means that another writer, equally deserving of their attention, gets short-changed. They also have accounting departments to answer to, and profit and loss to balance. And someone (not the acquiring editor) is going to have to be paid to copy-edit your book if they do decide to publish it. If the copy-editing is going to take two or three times as long, the copy-editor (who is almost certainly going to be a freelancer, not someone on salary) is going to expect to be paid for their time. All this goes into the equation of whether or not they can even afford to publish your book.
I don't mean to sound unsympathetic to your explanations of why you can't turn in a perfectly polished piece of work -- but the fact is, in the end none of these reasons matter. If you want to sell your book, you will almost certainly have to find a way around these obstacles and turn in a professional looking manuscript.
Consider it a test of your desire, or paying your dues, or an initiation, or (if you choose) look at it in a more negative light -- just know that there are other first-time writers who face similiar limitations who can and will turn in polished manuscripts, so inexperience really doesn't count as an excuse.
(As for fear of rejection, that isn't going to go away -- ever. That's another obstacle you need to learn to navigate around.)
