Georgina,
Fellow NT here (INTJ/INTP, depending on the test). I recommend that you find a supportive network of friends who you can talk to about the small problems, like staying on task. In that sense it was a very good decision for you to reach out like this.
What you're describing is something I deal with too, which is a tendency to use novelty (the thrill of starting something new) as a way of self-medicating to avoid fear.
There are a couple ways to deal with different parts of your struggle. Some possible options;
1) Block yourself from viewing your Medium statistics. It is rewarding to see how your story does but the feeling of reward is so strong that it can become compulsive and overwhelm more natural rewards like the joy of doing a good job.
2) Like life, take your stories one turn at a time. Pick one. Stay with that story only until it is done. You will have impulses to write other stories; create an outlet for those. Keep a book of ideas, and when you're working on a story, every time you have good ideas write them in the book.
If you have lots of ideas then you will probably write them in the book so much that you get frustrated that you're not making progress on the main story you're supposed to be working on. That's great; use the frustration to help you keep writing your main story.
3) Commit to drafting. Most writers only write one draft. This means they are under pressure to do everything correctly on the first draft. That's part of why they have writers block; in their mind they have to either write the next part perfectly or not write it at all. So they won't up retreating until they feel calm enough to come back. Then the cycle repeats itself.
If you commit to writing a draft then you have a new option, which is to forgive yourself and proceed humbly, knowing you're going to write something that is bad. Then, when you come back for a second draft, you can work on making it better. And you can repeat that process until you are happy. Most of my stories require three or four drafts. Yours may take less.
This process feels like it will take more time until you try it. Once you have done it a couple times, however, it takes off so much pressure that it doesn't feel like more work.
4) When I first got paid to write it made me feel like all of my writing had to be "official enough" to earn money. This is a big problem because it meant that suddenly I had all of these ideas about what my writing should be, interfering with my ability to write.
The thing that helped me was to commit to a higher principle that I chose for my own writing, and then decide that all of those other voices and judgments were unimportant and untrue unless they helped me achieve my higher principle.
The higher principle can be whatever you want. It can be a cause that is important to you, or it can be a virtue that you care about, or a goal that you have. Mine was learning and self discovery.
Whenever I felt paralyzed, like my writing wasn't good enough to earn money, then I used my higher principle to help me move forward.
When I started feeling like my work wasn't worthy and wasn't good enough to make money, I stopped and asked myself "Is it helping me to clarify my thoughts on something important to me?" And if the answer was yes, then I asked "Am I willing to keep doing that even if it means that my writing is not good enough to make money?"
The answer to that was usually "yes," and it eliminated my conflict to the point where I rarely get bothered about the money thing any more. I still care about it, but it is subordinate to my higher goal. Mostly, my concerns about money serve to remind me that I should make sure my writing is clear and accessible to others. If I am going to write for the joy of exploring and learning then I owe it to myself to figure out how to make it as engaging as possible to other people. But if I ever get so paralyzed by what others think that I cannot move forward, I can remind myself about why I'm truly writing, and that's enough for me to forgive myself and move forward
