I have a friend who often relates a similar story to me. (The friend is not me, but has been in the past!) As always, as detailed as this is, it's never the whole story.
My friend will be playing and winning at a reasonable rate, then the reverse... at a reasonable rate. When they're winning they often attribute it to the quality of their ability, and when they lose they often attribute it to bad luck. A common bias. Their particular story often involves relating what they've seen in High Stakes TV shows, and stating how the decisions presented are practically identical to their own. Repeating actions & possessing skill are very different.
You may be reading, and indeed doing what these books suggest, but it's likely that you're not digging into the concepts & heuristics that are the foundations of the decisions. For example, a plain hand history often won't retell the past play, opponent dynamics or skillset, whether Hero has balanced his lines, etc.
Of course, the truth is that what might appear to be true, only scratches the surface of the decision. Emulation is not mastery. Hard rules are good for processes you deeply understand, but ask yourself: Are you really there yet? and whether liberating yourself to explore is better at this stage of your development? It often is at all stages. Tiger Woods (Golf) changed his technique when he was world no.1, as did Phil Taylor (Darts). Just two of many examples. Poker can likely be classed a discrete skill like these sports.
I'd suggest a couple of things, each has helped me a great deal.
The first is focusing on one concept at a time, and I do mean concept. Your working memory is finite, don't overload it. Sure, read the books all the way through, but remind yourself that they're for education, not entertainment. Go back to the beginning and study the examples, paying attention to the concept. Work with it, test it, manipulate it in ways you think are wrong and right just to experiment.
The second is, do more to learn how to learn. Meta-Learning. This might've been the first point but it makes sense as an example of how to tackle point one. People commonly read, watch, listen to so much that there's a fear of missing out if you don't take it all in. They take an example, read it once or twice, and then move to the next with the full intention of applying the ideas presented. What actually happens is, because of the amount of new tools to play with, it ends up being emulation and not an attempt at mastery. Look at why you get a result from doing "this & that", and less on the how to get a result from doing "this & that". If you do, you'll also find out why not!
As a personal example, starting with the bigger fundamental concepts like position, value bet vs bluff bet, I went through the first "Easy Game Vol 1 by Andrew Seidman" book, chapter by chapter doing this, applying what I thought each concept was at increasingly detailed levels. I soon found out that, when I moved a chapter further on, what I thought I knew wasn't actually clear. And so I went back a chapter. Rinse, repeat.
I hope this helps a bit. As a side note, I thought this was interesting when you mentioned "My style was pretty much supertight until I started winning and then loose until I started losing". Perhaps you were winning because you were supertight, and changing to a looser game actually hurt your game. I'd be digging into why one worked better than the other.
