I'm familiar with this. There's no magic solution, sadly, but I'll share one thing that has helped me and may apply at least to some of your situations. I realised a while ago that my main issue was with nebulous tasks -- that is, the more concrete, defined, and _meetable_ a task was, the less trouble I had with it. So I started to break down large tasks, which never got started much less finished, into smaller ones,…
Lots of tools are great for this too, I've used Tomboy, OneNote and org-mode, all of which do this very well.