I have a gratitude journal in app form (Presently) which lets you know how long you've recorded on the app for (e.g. 25 days of gratitude, 100 days of gratitude) - pretty nifty. The generic prompt is, "What are you grateful for?" but the app also gives you hundreds of prompts that you can use. It also has a pretty decent UI and the colours can be changed which is nice.
I've pretty much had the same outcome as you, Owlbird! Some days when everything goes to shit, I sort of dread having to do the exercise but I've noticed that I've paid attention attention to positive things slightly more than usual which does lift my mood a little. With the toxic positivity thing, the gratitude journaling does encourage you to focus on the positives but I've found it to be a supplementary sort of tool rather than changing my entire way of thinking to solely find the positive in everything
I also think for there to be any benefits from gratitude journalling, it should be kept at for a little while to the point where it becomes a habit.