This advice may not be helpful as I'm not a person with ADHD (though I've struggled with deep procrastination), but journalling (and most other regular tasks) can become more difficult if you see it as a task that you "must" do for self-betterment.
I would focus on motivation to start (it can relieve the emotional pain from having negative thoughts cloud your mind all the time), commit to imperfection (just half a page is fine), and reduce small friction in the way (e.g. keep an open notebook in an easy-to-access place with a pen readily available).
Also consider making the act of writing a pleasurable experience. This can be done with a nice pen, doesn't need to be fountain, and high-quality paper. You can also be honest with yourself about whether it's helpful in your experience; you can feel free to stop if it becomes something you dread, versus something you enjoy and look forward to (or at least feel neutral about in tougher times).
I would focus on motivation to start (it can relieve the emotional pain from having negative thoughts cloud your mind all the time), commit to imperfection (just half a page is fine), and reduce small friction in the way (e.g. keep an open notebook in an easy-to-access place with a pen readily available).
Also consider making the act of writing a pleasurable experience. This can be done with a nice pen, doesn't need to be fountain, and high-quality paper. You can also be honest with yourself about whether it's helpful in your experience; you can feel free to stop if it becomes something you dread, versus something you enjoy and look forward to (or at least feel neutral about in tougher times).