Prioritize ruthlessly. Very few ideas can even be examined, let alone pursued.

Productivity + meta: Learn to be an effective Red Team, and use this ability on your own ideas and plans. 

Motivation: Find a way to remind yourself about what you care about (and if needed, why you care about it). This could manifest in any way that works for your. A post-it could be useful. A calendar notification. A standing meeting with colleagues where you do a moment of reflection (a technique that I've seen used to great effect at the Human Diagnosis Project). A list of recitations embedded among TODO list items (my personal technique). 

Allocate some time to "meta", like studying habit formation and self-management. For starters I might recommend Atomic Habits and some of Cal Newport's work.

I'm not an independent researcher, so this advice is probably less trustworthy than others', but I am currently on somewhat of an independent research stint to work on ELK, and have been annoyed at motivation being hard to conjure sometimes. 

I've been thinking about what causes motivation (e.g. thinking about various anecdata in my life) and I've also just begun tracking my time practically to the minute in the hope of this causing me to reflect on the sequence of stimuli, actions, and feelings I have throughout the day/week such that I can deduce any tractable levers on my own motivation. Though it seems too early to tell whether the time tracking will be fruitful in the end -- we will see.

An example of how "reflecting on the sequence of stimuli, actions, and feeling" could be helpful: today, I hypothesized that I was much more productive on two recent plane rides than I am usually due to being away from people and being action/motion restricted. And so I tried getting on a train; though I noticed I didn't want to work on ELK due to being anxious and hypothesized that perhaps my brain still wanted to pay attention to things I had been doing before getting on the train, and that, maybe, an additional reason I am productive on planes is due to security lines giving me time to reset my brain; I then tried resting for 20 minutes, to see if my anxiety would go away; unfortunately it didn't, though I then went on to think of more testable hypotheses and decided to lower my caffeine dosage [I had had about 120mg that morning, and caffeine seems to cause anxiety]).