I wasn't sure what kind of lack of motivation so I wrote out multiple answers

"I have an idea I really want to write but I can't bring myself to actually put words on the damn page" - this is probably the most ADHD brain-specific answer here but I force myself to write as Literally The Very First Thing I Do When I Wake Up. before I even GET OUT OF BED. I reach for my phone and make myself write until I've gotten through at least a couple paragraphs. I also employ the opposite tactic and try to make myself write last thing before I go to sleep but that one's a bit more tenuous as I've gotten better at falling asleep without noticing these days. I also use self-imposed deadlines (like u,u's weekly update schedule) (or externally-imposed deadlines, like exchange events). it's also possible to use other markers besides sleep, for example sometimes before or after I get out of the shower I will make myself write at least a couple sentences before letting myself do anything else, or in the past I have sat in the car after getting off work and written a few paragraphs before driving home (tragically can no longer pull this trick as I no longer have/use a car). however writing first thing in the morning literally before anything else is in my experience the timeframe when I have the least mental resistance and can generally get the most progress in, and also makes it more likely that I may be able to go back later in the day and write more with less mental resistance as well. this trick does depend on my ability to write on my phone though. for people who can't do that, maybe a bedside notebook?

"I have lost steam on this piece in specific and no longer feel interested in or attached to it" - for me this is usually because I've lost track of what I'm doing with it. I almost always write to a specific scene I have in mind that I need build up to get to... so the problem is either that I don't have a scene like that, or that I don't have a strong sense of what I need to lay down to get to it. sometimes I can't overcome this one tbh... I think ultimately my best bet is to fall back on my affection for the characters and think about what parts of them I like most and want to show off to other people. like look at my wife. look at how they respond in these specific ways to these kinds of circumstances. aren't they so cute. I need to theme this whole fic around this part of them I think is cute. this advice might only be useful to people who approach writing with the same extremely specific mindset as me

"I can't come up with ideas" - I look for prompt lists and throw several of them in a pot and try to figure out how to fit them together. it's like a puzzle. or a math equation. how do I write a story for these characters with coffee shop, stalking, and vampires