I've written a piece about this on the book's page on this website but wanted to talk here about how important it is not to give up when it comes to writing.
I couldn't tell you how often I came across that advice when I was starting out. It's all too easy to think, when all that is coming in are the rejections, "oh yeah?" when some published author somewhere says this but they are right. The keys to success are persistence and learning to improve your work all the time by submitting it and taking to heart feedback received on it. Always pay careful attention when more than one person says the same thing about your work too.
Beta readers are so useful because you can be too close to your own work to read it objectively, especially if you have gone through it time and again editing and improving it. I would also say support your independent presses as they give writers a voice the bigger, more traditional firms are unlikely to take a chance on. The indies play a wonderful role in ensuring there is more variety out there. And good luck, we all need a bit of that too, but hard work and persistence are more important.