i think the thing that has worked best for me is taking the tasks i need to do and splitting them into the smallest steps possible, and just do at least one of those subtasks (ideally as many as i can manage) and then grabbing my brain by the collar and informing it that i will feel satisfied with myself today because i did a task. And if i really cannot manage to split up tasks and do anything, then at least invent some task you can do even if it's technically pointless, like doing one (1) situp or whatever.

Eventually it has become second nature to split up the tasks like this, and my brain has begrudgingly accepted that it's supposed to feel rewarding.