Did you learn them in cluster or were they separated enough?
From my own experience, trying not to put too many synonyms at once can sometimes help.
The idea is that when you’re learning something, and you forget, it’s because your brain probably decided that this information wasn’t that useful in the first place, maybe because of a lack of exposure, but in my own little theory, also because something very close is already present and is already difficult to remember.
I think it’s also one of the big reasons why Bunpro is probably the “worst place” in terms of “synonyms hell”, since all the variations of a certain “way of saying something” is clustered as a chapter. Textbooks like Genki for example, will make you learn てもいい, then later on ても, to avoid clustering too many things and creating too much noise in your brain. The less noise, the most peacefully the information will be able to crystallize in your long term memory and how easy you’ll subconsciously retrieve it.
But right now, whatever the situation is, you struggle to differentiate them. When that happens, I think it’s pretty important to really put them side by side and examine them closely.
離れる (はなれる) : Same reading as 離す
別れる (わかれる) : Remember 別=分 in this context. 別に (べつに) means separately. 分 means portion. You create portion by dividing, same idea, same reading.
分かれる(わかれる) : Easy, it’s the same reading than for 分かる.
割れる (われる) : We already have 2 things we know that mean to separate and the reading start with わ ? Let’s remember this as the third わ of the family. Why 離れる isn’t a わ ? It’s not really dividing something that was part of a whole, it is two things that were together that drifted away, it’s not really separate, it’s more like “released”, “drifting away”.
PS : all my explanations are mostly made-up, and might absolutely not work with you. The goal is not really to give you a perfect explanation, but just to show you a bit how I do to differ those initially.
Once you can more or less differentiate them with a good confidence level, even if it takes maybe 10 seconds to do it, with time, and based on those good steps, you’ll be more and more able to just click faster.
But my big big big takeaway is this : Don’t go straight into the wall again and again hoping that at some point something will just change by itself. When a situation like this occurs, you basically have to find some strategy to differentiate them, which at first will cost you time and won’t make you able to read them very fast, but then, by iteration, speed will come. Focus first on being able to differentiate them, even if it’s slow.
There will be a lot of similar cases that look alike but with completely different meanings (縁、線、緑) or with reading that will even be different from compounds to compounds (黒髪(かみ)、赤髪(がみ)、頭髪(はつ)、銀髪(ぱつ)). Sometimes, things just sound “nicer” in a certain way, that’s the big perk of immersion, you can develop affinities with reading based on “what you’re used to hear” without having to create mnemonic based on textual forms. For example, I always mixed for とにかく “anyway” or “by the way”, but by watching Violet Evergarden, the male protagonist was using a lot とにかく to change topics (“anyway”), so now it just “feels” better to my ears.
Another counter-intuitive strategy is to learn more words, related to the previous one. Compounds can work marvelously but are never part of any core decks or website. Take 商工会議所 (Chamber of Commerce and Industry), 無人販売所 (Vending Machine). They are quite long, but they use easy kanjis, easy readings, and they can play the roles of “anchors” to remember some inner kanji readings, meanings, etc. I really like adding long compounds in my deck to have some “drills” for long successions of easy readings.
Don’t get too much frustrated with Japanese, there are a lot of similar problem with english. Why is it aquarium and not water-rium, hydrorium ? If we used kanjis, 水 would also have different readings in English. And we know if it’s aquarium, hyprodphobic or waterslide even without having to read them, we know because with time and exposure, we encountered them. So while it can feel overwhelming, with time, exposure, and curiosity, all those things will solve by themselves.
But once again, the main point : Don’t hit the same wall again and again, find new angles to attack the problem 
