There are lots of reasons why a writer might feel discouraged. Rejection is a big one. Writer's block (or the phenomena that pass for it) might be another. I personally have a difficult time with productivity, or to be specific, how little of it I feel I have.
I'm not fair to myself. Does this sound familiar?
Writers have different writing styles, and they have different writing processes. My process happens to involve an enormous amount of background work for each story, followed by planning and scene-sketching - for the whole story if it's a short story, and for at least a section of several chapters if it's a novel. My plan can flex, but I like to know where I'm going.
Sometimes it's a good thing that I have lots of background work to do, because this kind of work is stuff I can do while I'm doing other things. And I have lots of other things to do. Typically I get three hours a day to write, five days a week. The hardest part is that I can't always count on having this time - unpredictable things like illness will derail my process for several days at a time.
I write scene by scene. When I have a draft, I get critique. I usually then have to take the draft apart on first revision and change something major (how major depends on the story). I write stories that go somewhere, that have character arcs for multiple characters. It takes me forever. I watch people around me talking about all their story submissions and/or acceptances and I know that's not something I'll be able to achieve for a number of years, or possibly ever. Even if my time goes up, I'm still not going to change into a flash fiction writer overnight.
I imagine though that other people experience different kinds of frustrations with their productivity. Like writing a whole bunch of stories and not having them land anywhere. Or having to trunk things because they don't know what to do with them or where to sell them.
The thing is, the Muse works differently for different people. I find if I try to change my style, I can't function at all. To some extent you have to go with that.
On the other hand, I have learned some things that help. One is that I've learned to keep my Muse awake - i.e. not to lose my drive and inspiration - by making sure to do at least one writing thing every day. That includes pulling out what I've written last and looking over sections of it. This makes it tons easier to resume what I was doing when the free time presents itself.
I also had some really great advice the other day from my friend Deborah Ross (of Darkover fame). I'll paraphrase what she said. She told me when you feel like your productivity is down, you may be counting the wrong kinds of progress. Getting a story finished is wonderful, but if that's the only kind of achievement that counts, you'll spend a lot of time frustrated. Just keeping the Muse awake should count as progress. Each sentence you write should count as progress. The important elements of worldbuilding and planning should not go unrecognized on the progress-meter either. All of these things contribute.
It was really good advice. I can't say I've stopped feeling frustrated by the unpredictability of my writing time, but counting progress differently has helped a lot - so just in case you've been feeling frustrated too, I thought I should pass this on.