My mind has a groove 'it' decides on, and that's where I go - but it's bloody annoying.
Being able to switch mindsets at will would be fantastic!

This is slightly different and like you say it is difficult because of 'attention slew'. The only way I can switch mindsets regarding two very different subjects is to give at least a 45 minute break between the two, and then to employ some of the techniques described above.

Next time you find that you are thinking about one when you want to write about the other, sit down and write 3 or 4 paragraphs of what you're thinking, perhaps more if the feeling takes you.

Then once you've come to an end say out loud some words along the lines of; Pause; I will come back to that later.

Then shut the computer and walk away, 1.5 hours is ideal however you may not have that much time. So spend 20-25 mins switched off, go for a walk in the park, spend some time near trees and nature in general.

For the last 20-25 minutes start some productive meditation, say to yourself words along the lines of; right I'm going to think about xyz now, in particular abc.

After a couple of weeks of doing this you'll find that you switch much more naturally between mindsets :-)

Cg