Unfortunately the annoying thing is there really isn't a way around the two Big Actions of well, Making stuff (whether it ends up Good or not is irrelevant) and Posting it/sharing it. Which is all Easier Said than Done but here are my thoughts on it.

As long as ideas are in your head, they don't actually exist as works, stories or art, but more the idea of an art existing unspecifically. When you write or draw something down, it's frustrating because you're forcing yourself to make a Decision. Ideas must be translated into words, or lines/pigment on paper. This stage, of working with the Tangible, is where a lot of frustration but also improvement happens, because then you actually have something to work with, respond to, etc.

My tip for art is that if you're stuck, do STUDIES, copies, from life, reference photos, or fave artists. Get used to the physical motion and action of making things, and the feel of your materials. This will break down "resistance" to starting or knowing where to start because your instructions are are already in front of you. It doesn't need to be pretty or perfect, but the important part is moving out of the Thinking/imagination only stage to the Tangible stage.

While drawing for me is very intuitive, writing is a real struggle XD but I've slowly sort of been getting better at doing more this past year. I write on my phone, in my docs/Google note, in designated notebooks. I end up scrapping a lot of stuff but it's good to have on hand so you can look back if you need to. I think there are also ways of doing "studies" with writing you like, copying, transcribing, reading and taking note of writers you like and how they do things. I'm still working on this too so we're both in it together.

Sharing work:
It's a dual process of reminding yourself that you should be creating For Yourself and your pleasure and not just a fickle audience; however, art IS communication and you do need to do some presentation if u want people to know what you're about. AO3 original stuff is good, if you write anything with specific tropes or kinks it's good to tag so people who like the things you write about can find them. I think it's easier for porn bc of how Direct it is, maybe less so for more narrative stuff. If you work in a specific genre–fantasy, sci-fi, mystery romance etc–or even not fiction-specific, but if about a niche topic that pertains to your work--see if you can find communities that specialize in that genre and post there. But that's the next step of the process of "Make Things" , it's "Describe what you made," be able to talk about your work so you can connect with people. It's all complex and easier said than done but hope this is a bit helpful.