Me too. What's helped me a lot is to just relentlessly make "what counts as starting" smaller and smaller until it's doable. Where "file taxes" is very very hard, "look through email and remember name of my accountant" (and many other TINY tasks...) is much more doable!
I've noticed a lot of times what appears to be procrastination is actually uncertainty as to how to proceed. The mind recoils from it's own inadequacy and just labels the whole thing "bad". An advantage of breaking it down as you say is that it pushes the uncertainty a step further away. You may not know what's required to do your taxes, but you know what's required to search your email. Many times, the way to perform the next step is produced by performing the current one (e.g. your accountant tells you what documents are needed) so the overall task is not nearly as painful as you anticipated.
these tasks are similar to the 'intellgent dumbing down' talked about in Getting Things Done, no wonder they work!
Having said that...I'm not diagnosed as ADHD but I definitely feel I exhibit executive dysfunction symptoms, and one of my siblings was diagnosed, so I'm feeling confident I'm ADHD adjacent at least...and just trying to implement Getting Things Done helped me immensely. Not only does it help with having a less stressful day (no longer do I have "what am I missing? what am I forgetting? Ahhh, angst!" feelings), it also helps me to just stay on task and/or when I switch tasks, know that I have the confidence to come back to wherever I was.
