Why can I only seem to write when upset/distressed?

It is your style. You might have a block. You can go with it.

Ever notice that the songs and movies people seem to connect with most strongly are about broken hearts? Whether or not they’re caused by ended relationships, death, or loss in general, the ones which have the biggest impact tend to be the saddest. So, my point is that you’re not alone. ;)

Writing could simply be your outlet when you are upset. There’s nothing wrong with that. If it’s cathartic, go with it, even when it seems negative. It’s a good release.

I find this myself in relation to creative writing. It’s much easier for me to write when I’m going through some sort of intense, emotional experience. So, I’m in the first throws of love. I’ve lost a loved one. I’ve been in a big argument. I think that’s pretty normal and as @DrasticDreamer suggests, it can be cathartic.

What about keeping a feelings journal? That you use to vent your feelings through your writing at these times? It could then act as a tool when you want to draw on those experiences in a creative piece. I don’t just mean when negative things are happening, but when something great occurs. You have a success with your art. Your child does something amazing, cute or just looks at you and your heart melts. Your lover says something that makes you feel tender and/or adored. You go out to the front gate and the birds are singing and you realise how damn lucky you are to be alive. Your child drops around to see you with a bunch of flowers. You talk to some old person at the shops and their reflections on life make you laugh and feel happy. All of these have happened to me in recent times. Perhaps you can keep a notebook in your bag and when similar events occur, jot down how you are feeling.

Another idea is to use writing prompts. Perhaps choose a different positive prompt each morning or at night (whenever you find it easiest to write) and write 200 words (or whatever works for you) on that prompt. Only pick positive topics. I think it’s about developing the habit of looking at things from a positive perspective. And, you could also try to write in third-person. So step outside yourself and look at it from an outsider’s perspective.

Adrenalin is a powerful fuel.

Pain of any kind brings out more creativity.

John Lennon said that he used depression to help him be creative

Don’t listen to people who tell you that the best fiction is written by anguished and tormented artistes. It’s been repeatedly demonstrated that people write pretty consistently regardless of their mood and regardless of their feelings towards the work. Whether you think what you’ve produced is your magnum opus or the direst shit ever committed to page, chances are good that they read exactly the same to someone else. To quote Jack London, “You can’t wait for inspiration. You have to go after it with a club.”

It’s probably easier for you to write when distressed and miserable because it removes your desire to do anything else, thereby reducing distractions. I think you’ll find if you can develop the discipline to force yourself to write regardless of your mood, that you’ll produce work which is just as good no matter what your mood.

It sounds like you are using your writing as an outlet. You only feel compelled to use that outlet when you need it, which is in times of emotional turbulence.

Asking a rhetorical question, here. What are you being pessimistic or upset about?
The subject-matter can help you figure it out. If you have some writing intentions and it vears off track- when did it change towards the ‘dark side’ ?
Love? Life? $? Current events? Religion? Self-introspection? Family?.....
Figure out what you wish to convey. What message. Then write points or guidelines you want to include. Keep checking back on the points.
One way to make yourself begin and end less dark and angry, try writing the intro section, stanza, whatever. Then write the last or ending portion. Beginning and end. Sometimes it makes what is in between, easier.

I think pain is the fundamental of all kinds of arts.

So my opinion is, if you are happy when you are creating, that’s great; if you just feels like doing your job, well maybe a lot of people feel that way, too; and if you creates more in a not exactly the most positive mood, well, maybe that’s closer to what creation really is about at the end of the day. The truth is, as long as you are fine with your own way, then it’s perfect.

#fundamental -> foundation