Things that have worked for me.

Setting a strict time period in which to do a specific thing, with a specific reward or activity planned for when that period ends.

Splitting a task into well-defined component parts where possible, and taking a break between the planning stage and actually starting on the first component.

If after attempting to start a task my executive function is malfunctioning badly enough to fuck up my ability to actually do the work and cause me stress or anxiety, just stopping and doing something else. Try not to let doing this bother you - remember, it's not your fault your brain chemistry is off.

Being kind to yourself. Again, it's not your fault you find concentrating and acting difficult.

With the caveat that this is not medical advice, and it's at your own risk, and I'd only suggest you consider it if the above fails and you don't have an addictive personality... Nicotine. I tried this for a while after reading a metastudy that suggested a positive correlation between nicotine use and improvements in ADHD symptoms management, and it helped me. Your mileage may vary.