I'm going through a stressful period in my life, where my creativity is gone, and long projects overwhelm me.
I'm looking for ideas that will improve my writing skills, rather than express myself. I'd like something that will allow me to see myself making progress.
My creativity is just gone. My disorganization is at an all time high. Even planning and writing a flash fiction piece is overwhelming, right now.
I do a lot of free writing every day on recovery forums, so I get lots of practice in finding my voice. I want to make some measurable practice in technique, though.
90 minutes is too long and a story is just too creative and large of a project, though.
Prompts are good if they are an organized set, meant to be used in order, with clear cut goals that can be measured.
I'm putting out a decent stream of writing. I want to do something, though, that will improve my writing, in a noticeable way.
I understand the kind of thing you're asking for, but I think you need to be clear about what improving your writing means to you. Writing is so subjective that it's not a case of saying, "... and with that piece done, you're now a Level 4 character builder". I'm not making fun of you, just saying that you need to have a goal in mind before people can suggest drills that might help you reach it, and the ways of measuring your progress are going to depend on what you're holding your writing up against.
From what you've said, improvement may mean to you:
1. Being able to write increasingly longer pieces: perhaps beginning with a a paragraph describing something in particular, moving up to a conversation between two characters, to the outline of a scene, to a complete scene, to a piece of flash fiction, to a short story, to a novella, to a novel.
2. Writing for increasingly longer periods: Begin with a ten-minute session, using a timer, and increase by thirty seconds or a minute each day, up to a goal of 90 minutes.
3. Creativity: Begin with coming up with a basic character, with a goal. Then add an action they can take to try to achieve that goal. Then add them coming up against some obstacle. Then add something that they try to get through or around the obstacle. Show them achieving their goal. Then try it again, but with two obstacles. There have been some consequences from the first obstacle the character encountered, and they will play a part in the encounter with the second.
A book that has lots of exercises for practicing specific writing techniques is
The 3 A.M. Epiphany by Brian Kiteley. Working through them might work for you as a project with measurable goals: cross them off as you go through them. You could use them in conjunction with the timed sessions, perhaps.
I hope some of this helps.