I’d be interested in hearing more about this. I often get the “so much stuff to do that it’s overwhelming” feeling, which creates an unproductive feedback cycle.
For me, learning to delegate was a battle against my perfectionist tendencies. "But that's not they way I would have done it." Yup. When you delegate, people do it their way. To me the key was learning to parse out what is it that I care deeply about, and what not so much. Be sure the things you care deeply about are in the spec, and let go of the things that are not so important.
Make time for yourself. I am a ham radio operator, so for me spending a few days in the sun butchering a pile of aluminum tube to make antennas cures many ills. That is unlikely to be a universal prescription -- find yours.
Saying "no" is a forgotten method of self-preservation.
Similar to what the other user mentioned, I get moving again by focusing on the easy tasks to feel productive again. I make a simple text file with everything I need to do, sorted by month, and each month sorted with the easiest to hardest tasks.
I write every single thing on my mind. For example, I need to order something online, so that gets added to the list. I have a wedding to attend this month and need a new white shirt, that gets added to the list. I need to wish someone happy birthday next week, so I'll add that to the list. Do I need to book a haircut appointment? On the list it goes. Everything goes on the list. It really helps to clear my mind when I don't have to remember these things.
Then, when I start my day, it's not an overwhelming number of thoughts. I look at the list for this month, and I just choose a task I'm in the mood to finish. Do I want to check off a few easy ones? Do I have the entire day to myself and I can focus on a more complex one? If I'm going to the city, then I can take care of all the ones located downtown.
In short, write everything down, start checking things off the list.
I do this by keeping my list on a small paper (A6) in my notebook. When the page is full, I transfer over only the incomplete tasks that are still relevant over to a new page. Often also refining their scope. This chance of default from 'keep' to 'not keep' has had large impact on how many (useless) tasks I drop, and the associated mental burden.
There's a mental pressure that results from having a number of things on the go at once, and writing each thing down relieves that pressure, probably in a similar way to the aforementioned ticking off of simple tasks.