Before getting medicated, I had a multi-step process for getting those things done. On a typical day I’d struggle, but I would have good day/days where I was able to feel motivated and get things done. So I would plan ahead for these good days.
Break the task down into as small of tasks as possible. For taxes or making appointments, this usually also means having a lot of documents ready and if there’s a phone call, an outline of what I want to go over during the conversation. Maybe some research on what the expected thing would be like if it’s a new thing.
Don’t try doing all of that at once, back to back. Just do each part, one or two at a time in the days (or weeks if there’s time) leading up to my deadline. Get all the docs together in one place. Look them over to make sure they’re all there and I understand them. Organize them in order of need. All separate tasks for separate days.
Then, when I hit a good productive day, knocking it out is much less overwhelming and draining because the tedious work is done. It’s just the action of the task that remains. It’s worked for years. I still do it without realizing it often. I think it’s just a good plan of action in general for everyone to makes tasks manageable.