Are you writing a novella, or a short story, or a novel? Many people who write a novel, write a novella first.
If you write a novel and it fails to find a publisher, or maybe is not up to your expectation, the process of moving on is easier. To have an unpublished novel can stress a writer, put them off even trying again sometimes. With a novella that is not a problem. You can walk away from a novella far more easily, if it doesn’t work.
In a sense you are practicing writing a novel. You may however know that what you have in mind lends itself specifically to the novella form – but it is not however a short form of a novel.
The novella is a specific form of its own with the following features – novellas often focus on one conflict do not have so many characters - they can often be faster paced as there is no time wasted on longer meandering style of writing - novellas can also be experimental as again, there is less at stake – and finally they can be high impact.
Are you writing a novella? There are so many writing situations for which it is perfect.