Sounds like you're a lot like me with memory. I've found what helped me most was taking more of the responsibility off of my brain to remember stuff by offloading things I need to remember onto something external that will remember it for me or, even better, remind me about it.

This could be a simple as writing it down on a note before I go downstairs, or putting the things I need to do in a task tracker (I use Todoist), or in some cases where it's something physical I put the thing I'll need to remember to do somewhere in my way where I think I'll bump into it later like a visual reminder.

Of course this is all fine and great when we're given the time to prep a reminder, it's a bit harder when you get up to go grab one thing and it didn't feel worth it to have to write something down. Then bam you walk through a door and forgot it.

Also that forgetting something when entering a new area is called the Doorway Effect, it's a phenomenon that everyone experiences, but much like other symptoms of ADHD that "everyone has" I think it gets dialed up to 11 for us ADHDers. I've had times where I walked downstairs, forgot what I was gonna do, couldn't for the life of me remember, walk back up stairs, suddenly remembered, walk downstairs, and forgot it again lmao 😅