It took two years and 25 rejections before my first book sold, but by that time I'd finished the second and begun the third.
Each time a rejection came in I looked over the sample chapters, picking them apart, worked to figure out what wasn't working, and tweaking the balance of the book. I got feedback from beta readers and did rewrites, 25 of them.
All this on a manual typewriter. I'd have killed to have the laptop and printer I use today, so no whinging about it being too much work. Nothing is too much when it comes to honing your craft.
The smartest thing I did was cut the first 5-10 pages to begin the book where something interesting is happening. That was two years into the go-round. We all kill our darlings. It's part of the job.
It got me a request and then an acceptance. I was over the moon until PW reported that the publisher went belly up in a bankruptcy. Good thing the contract hadn't arrived.
Then an agent told me I was a good writer but unpublishable. THAT made me furious, so I rewrote the whole book -- again -- and sent it off to the biggest dog on Publisher's Row.
It sold.
Two years of it, so pack up the impatience, this is a marathon, not a sprint. If you want a career as a professional writer with books in stores, focus on your CRAFT and write every day.
WRITING, telling a story that's burning inside you demanding to be shared is the goal. Selling it is just the cake under the frosting.