Hi! I wanted to give you some pointers based on my own experiences. As a background, I had interest in non-fiction almost from when I started reading seriously. Therefore, history was something that grabbed my attention. I eventually ended up with both a BA and an MA in history, along with a career as a librarian.

Obviously coming up with a field can be troublesome on an academic level, where the concept of a master’s program is to specialize, as opposed to a bachelor’s degree, where this can be less of an issue. One of the logical things to do is speak with your university history adviser for their own experience with this. I suspect that we all go through this at some point! It was my experience that history professors are very collegial with post-bachelor degree students who are expected to develop skills in working with others in the profession.

Here are some ideas:

• Think back over your own personal reading interests as you grew up. Are there similarities among the various topics? For example, are the events in the same country/time period? Are they interrelated by general subject, such as history of science/business, etc.?
• Consider courses you have already taken in your education. Which ones affected you most personally? Can you use them to develop a focus?
• Also consider the likelihood of finding primary sources in the area. Professional historians need to spend much of their time with original items. For example, do you speak foreign languages that might be related to your interests? Can you access material related to the topic without spending too much money on travel, copying, etc. More detailed research may be important in the future, but this may not be the point to start with a topic that might be troublesome at this point.
• What are your capstone requirements for your degree? Do you have to complete a full thesis paper/research paper? If so, you will need a faculty adviser for this. Examine the faculty for your institution for those professors that specialize in areas of interest to you and reach out to them.
• Finally, you need to develop good skills at concentration. You need to avoid going too far afield while you are making your ideas. When you are innately curious, you can get into trouble skipping around.

Hope this helps!

Greetings! In addition to the great points put forward by u/jwt0001 on the matter, I wanted to weigh in and give my tuppence on this question. For a bit of background information, I am definitely not quite at the level of "completed masters" or even "working" yet, still being a (very passionate) high school student in their final year. Still, I find that when it comes to choosing an area of history to specialise in, there are some general pointers which might be helpful at any level. Let's begin:

Firstly though, a note on this part:

[w]hether on[e] should worry about this in the first place

This of course depends on your Masters programme, and whether or not it requires you to choose a specific field to go further in-depth on later on in the course. If not, then it depends on what you want to do with your degree in the future. The job market for History degrees is...not great, to say the least, and I highly recommend this Monday Methods post from more qualified contributors and flairs about the relatively low employment prospects of a History PhD student. More often than not, the benefits of a History degree in the professional realm deal with the "soft skills" that you have developed from the course, and relevant fields which require proficiency in those skills. This might include legal postings (where the reading and interpretation of documents is a must) or the information sector (especially with libraries or museums). The post linked above has further examples, so highly worth a read as well!

Now then, moving onto how to choose an area of speciality within the masters course:

Is a particular time period that you have studied in the course of your masters (or will be studying) which has been more interesting than others? You mentioned in your post that you generally choose a wide range of course topics, so the exposure that you already have to the "surface-level" (if you will) landscape and historiography of various time periods should be a great starting point. Considering which course you were more interested in, naturally curious about, or seemed to invest more time into researching about might be a good way to narrow down (but not quite single out) which field you want to specialise in. I for instance, whilst definitely enjoying reading about the Middle Ages and Early Modern Europe, found it far easier and more interesting to delve down that "rabbit hole" of the 19th and 20th centuries (specifically the First World War). It is not a bad thing to be interested in more than one area of history, but more often than not you will find yourself naturally leaning towards areas which are just that bit more intriguing and fascinating. Which bring us to our next point...

How easily accessible are sources for the time period? As jwt0001 touched on in their comment, historians are expected to engage thoroughly with primary and secondary sources in their areas of expertise. Due to the nature of historical sources, some areas lend themselves to being much more accessible in terms of sources than others. Using my case as an example again, a lot of the secondary sources on the First World War are relatively cheap to purchase and easily found (not to mention a wealth of online articles and journal publications too). A fair amount of the primary sources (government correspondence, meeting notes, dispatches etc.) can be easily found via Google or on pre-scanned pdfs, so it makes the process of my research much simpler. Now, that does not mean that you should shy away from areas with sources which are not easily accessible, but it is a useful thing to keep in mind when deciding where to specialise.

It is always useful to get the advice of your professors and other teaching staff on the matter, so reach out to a few of them who specialise in areas which you might be interested in. Ask them questions about the process of their research, the nature of their sources, the historiography on the subject, and just about the overall appeal/difficulties of their historical fields.

Hope this helped, and feel free to pm me with any follow-ups or queries regarding additional advice as you see fit!