This is so so so important, seriously, write everything that comes to your mind, it'll help you a lot.
First of all, I'm no writer (at all lmao) but I'm working on a webcomic on my own so maybe I can help you out a bit?
I'd also advise you to try writing a kind of detailed summary of your story. Like, you said you have been thinking about your universe for at least 10 years right? You should have at least an idea of what you want your story to be like. Then try writing it down. Write it as badly as you need to, don't worry about it not being beautifully written, just try to "sketch" the story for yourself.
I don't know if this works for everyone, but it helps me a lot to actually write things down (or doodle scenes, that also helps) instead of having them floating in my head. They make my brain get to work and helps me see the exact things I need to work on. If you want to try this out, seriously, don't worry about beautiful adjectives or whatever, just go on and write it to see if the story works.
I have a lot of drafts about my story (as a whole and more in detail drafts of certain scenes). They don't even have dialogue, they just need to tell what happens and allow me to understand the story better.
After a long time, when I finally had the story pulished, I created a new text document and divided the whole story in chapters with little descriptions about what happened in each one. After that I created another one and started writing the first few chapters in as much detail as I could (now with dialogues and all that) - these are the documents I use when sketching the pages of the comic c: I don't necessarily stick to them 100%, I make as many changes as I need since the story doesn't work the same writen than drawn.
I... hope that made sense x'D this is my creative process, it has worked quite fine for me so I hope it helps you a bit. Oh! Also, if you can, try talking to a friend about your story! Try summarising the story to them, or whatever you need the most help with c: it'll make you think very hard about your story and your friends may have some cool ideas that could help you out <3
Best of lucks!!