Ask HN: How do you overcome the problems of task switching?

Hello! I am a software developer and over the years I found that I am extremely unproductive when it comes to finishing one task and then starting another right away. It is best noticeable when I complete something that took me more than one day and then I am guaranteed to not do anything productive for the rest of the last day(doing something for 3.5 days - unproductive for the half of the last day). Same applies to working on side projects. I am focused when I research one problem, implement it, but I have no willpower to continue to the next problem. This affects my career in a bad way and I am sure I am not unique and there are ways to fight it.