From what I understand, you asked four questions in one: 1. how do you learn, 2. How do you decide what to learn, 3. How do you manage your learning time and 4. How do you handle the pressure of that huge mountain of stuff you don't know. I will try to break it down a little. My Job requires me to rapidly understand fields I've never worked in and try to understand as much as possible in very little time, so I feel like I can contribute to answering it. I am not saying my thoughts to this topic are the best TM, or particularly well thought through, but it works for me.

1. How do you learn:
My learning strategy might seem a little weird but I'll explain it anyway. For me the first part about learning is about familiarity. If your brain sees to many words it does not know it subconsciously shuts down and you get frustrated/demotivated (at least for me). So you have to iterate over a topic in order to feel familiar with it's vocabulary. Just think of those Wikipedia rampages where you go deeper and deeper down certain words until you don't know where you originated from: that's because you are not familiar with the vocabulary of the field. So my first step is to learn the vocabulary of the topic by 1: reading a short book about the basics (or the first 100 pages or something) and 2: I find a place (online) where people working in this field hang out (e.g Reddit, HN etc.) and just read what kind of problems they have, which kind of words and tools they are using which kind of projects they work on. I do this daily, multiple times, and follow on things that really spark my interest and try to understand as much as possible. My brain works very interest based. I try to answer simple questions in forums, and try to get involved but only in simple stuff (since obviously I am just learning). At this point I try to apply the very basic things i've learned and iterate myself further by asking basic questions about stuff I don't understand etc. After having reached a certain familiarity with vocabulary and basics I try to explain why does tool X exist, what problem does it solve, what pros and cons exist. I think about how I would explain the necessity of X to someone who is not in the field. After that in the second phase I learn mostly like everybody else from books, online resources and just doing what I learn. But now I can read books a lot faster and with much fewer frustration because my brain is familiar with it, knows why X exists and which cool projects X is applied to. Thinking about it, my strategy is mostly tricking my brain into not being bored or overwhelmed.

2. How do you decide what to learn:
I learn because my job requires me to be familiar with Y. That's one part, can't really change much about that. The second part, for me personally is purely interest based. I always try to learn concepts/methods instead of tools. Don't care if it is useful or you will ever really apply it, BUT: if I learned a useful concept instead of a tool, I will always be ready to apply it somewhere else. We humans are masters in generalizing things and applying concepts we have seen somewhere else.

Since you asked about HYPE-TECH-A vs something that truly interests you: I always in my life picked my interests not the hype, and it always worked out. If you are motivated to learn something you can gain knowledge several times faster compared to force feeding yourself something that you might apply maybe somewhere in the future.

4. How do you handle the pressure of that huge mountain of stuff you don't know:

Just have a good mental health. Be aware that staring to long into the abyss of stuff you don't know will never lead to anything good. You have to be aware of the things that you don't know, but let it give you a joyful humbleness instead of fear. Just think about it this way: you will never run out of interesting things to learn. The joy of learning will always be available. Your mind is not a commodity of your future employer, instead learning new things should be your privilege and bring you joy.