Well... I was a writer by profession, but not that kind. I've written every possible sort of commercial material, from simple brochures to musical video productions, but I've always shied away from fiction for the same reason Linda gave -- it's too true!
Around 1980, a former English teacher of mine published what's called a "regency romance," and I went to her booksigning. She autographed my copy with, "If I can do this, so can you." I knew she was right -- but I had zero interest in the genre.
But about six years later, desperate to buy my first computer, I was casting about for a way to earn a few thousand dollars in one swell foop... and just like in the movies, I heard my old teacher's voice in my head, all echoey: "If I...I...I... can do this...this...this... so can you... you... you..."
Well, I did some research and found that when they call those series romance novels "formulaic," they aren't kidding: some publishers actually have certain page numbers
by which certain events in the story have to happen! It seemed awfully rigid to me... but at that time, a new writer could expect to make about $4500 on a series romance, and that was about what I needed, so I decided to try it.
But my story (titled "Floodwater") -- quickly became too dark and intense for series romance, and it wouldn't fit the pattern precisely. It was too heavy for romance, but not quite complex enough for mainstream fiction, y'know? I wrote the beginning, the ending, and key scenes throughout, but it's only about halfway done, and I'm sure I'll never finish it now.
I liked it, though! Until I wrote it, I was convinced that I couldn't write fiction... so it was a good experience, and it showed me that I could
So... based on what I learned during that time, my primary suggestion to you is to approach the project methodically. If you were writing mainstream fiction, you could run with whatever ideas came to mind -- but the romance market is fairly inflexible.
You'll want to read romances from various publishers, then request a writer's guide from each publisher you'd like to write for. Study the guide and structure your story to fit it. You'll probably want to outline the story, event by event, scene by scene, and make an index card for every scene (noting the characters, location, and what happens in the scene), so you can easily adjust your structure as you go.
Go to booksignings every chance you get and talk with the authors, especially if they're writing in your genre. If there's a writer's group in your area, where publishing novelists get together and talk, see if you can join. Before you decide to share your work with them, though, consider: a lot of writers feel that it's better to keep the book to yourself until you're nearly finished. It's difficult enough to keep the momentum going when you're writing a long project -- if someone gives you a negative (or worse, indifferent!) response, you can lose heart altogether.
Finally, about this "it's been done before" problem: as others have said, you don't need to worry much about that. In romance novels, at least, everything
has been done before! What makes a story interesting is not just what happens, but how
it happens, where
it happens, who
it happens to, and what else
is going on while it's happening. So don't let plot similarities scare you off.
I guess that's all I know. Good luck!