Your question specifically called out 'learning' as opposed to 'focus', so I'm going to offer a slightly different take to what I've read below. (all great advice, especially daily management)
I've found the biggest factor is having a clear picture of what I'm trying to learn, with a practical use case / example to work on, which ultimately becomes my reference point.
Learning then becomes a process of making the example work to achieve the expected result. A series of problems I can tackle, using my "mental skillness" rather than fighting it.
There is overhead in context switching, but I balance that with the mental effort of fighting distraction.
If I feel like I'm struggling to focus on my current task (say, reading about "High Availability and Failover across a set of database nodes"), I'll switch to a different task that's still related to getting my example working (maybe draw some diagrams to visualize the pieces I've learned about so far, or set up a few servers and install the database software ready to configure, etc...).
The trick is to try to make constant progress relevant to what you're specifically trying to learn, but vary the sort of stimulus you're feeding your brain to keep yourself engaged.
I've come to see my subconscious as my personal OS:
'struggling to focus' is like blocking that thread on I/O or some mutex lock. I see it as an indicator to give some CPU time to some other thread (ideally all part of the same high priority 'process'), flush the buffers, etc...