If you’re finding it slow going I wonder if it might help to concentrate on one story at a time? There are plenty of things that will try to stop you getting your story finished, but your other story shouldn’t be one of them.
Seconded.
As a young(er) man, I'd generally start something, have the mad burst of enthusiasm burn itself out (as it always does), then skip to the next thing in the hope that absence would rekindle enthusiasm for the first project. It never did, and of course by the time I went back to it, everything looked dead on the page. So then I'd start over, have the first mad burst of enthusiasm (because it was technically another new project by then, going in a different direction, etc), move onto something else...and so on.
At some point you (generic 'you') have to learn to settle to it and you have to learn to grind. If ideas for other stories occur to you, make a note of it, but don't let it pull you away from what the work already in progress. Also, remember and accept that it's OK if the first draft is written in a rush, ridiculously baggy, and generally messy as hell. No one else ever has to see it (certainly not an editor or agent!), and that's what second and third drafts (edits, if you like) are for.
As the saying goes (and yep, it's a cliche): Don't get it right, get it written.
As for right now, choose a project and stick to it. And one way to choose is to ask yourself this question: If you were told you were going to get hit by a bus within the next 24 hours, which is the story/idea that you would want to be remembered for?
When I finally asked myself that, the answer was obvious...and the novel was completed within eight weeks (writing for two hours solid after work, two or three days a week, and for three hours on a couple of Sundays)*. I then moved on to the other project that had been burning a hole in my mind, and you know what? All the sticking points seemed to have resolved themselves ahead of time, and the ideas I'd had - old and new - had a clarity they'd always lacked before. They just fit. I wrote that second novel in just over six weeks, then moved on to what turned out to be a short story (then another) while I worked on redrafting the first novel.
* If that seems quick, the maths is pretty simple. I was averaging around 40 wpm in those days. 40 wpm * 120 mins = 4,800 words per day. 4800*3 sessions (avg per wk) = 14,400 words per week. 14400*8 weeks = 115,200 words in total. 14400*6 weeks = 86,400.
(And yep, I'm faster now and I get to work as much or as little as I like (which is dictated by the work itself), but I'm also more easily distracted.
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