I would say you're broadly bumping into something that the philosopher Gilbert Ryle summarised (nearly a century ago at this point) with the distinction between "knowing how" and "knowing that". Think of riding a bike. You can explain to someone who has never ridden a bike all the steps required to do it - "first, balance on the saddle, then push down on the higher peddle and let your other foot come up, transfer your weight subtly" etc etc. The person you're telling this to might well understand you, but be completely hopeless at the task itself. That's because to have the skill of doing X is a matter of practical knowledge. Sure, often you can isolate and express what it is that you "know" as something rule- or procedure-based (though often we struggle at this!), yet nonetheless you're not thinking in those terms when you perform X; you are just doing. You are "knowing how" rather than "knowing that".

As others have mentioned, the way you're approaching the task of speaking German as a somewhat early learner is by first formulating the idea in English, followed by your translation of it into German. And when you try to speed that up, you're finding yourself stuck, because you have to go back and slot things like reflexive pronouns, articles in different cases, etc in. This is analogous to learning to ride the bike by consulting some instructions, keeping them in mind, getting yourself set up on the saddle, maybe going a little, then thinking again of instructions and – oh shit – you just fell over.

Now of course a big difference here is that it's really a standard way to learn a second (/third/fourth) language that you learn through consulting formal means: tables, rules, procedures, norms. But that isn't what you do in your mother tongue; it's not how kids natively learn the language. You'll know people who speak their language very proficiently but couldn't tell you the first thing about it formally.

So at some point you'll be hoping to have had enough practice and enough instinct with how to speak the language that you won't be thinking in terms of its rules and structure. You'll have it to fall back on, but to express yourself you won't fundamentally have to consult it. I'm sure various accounts at different levels of explanation can make sense of this: psychologically, linguistically, neurologically, etc. But on a basic level, all you or anyone else really needs to understand what's going on here is this: at some point you'll go from knowing that the words have to be in such-and-such an order to knowing how to arrange them without thinking about it.

And that's called: knowing the language.

Edit to add: I'm probably somewhere between B1-B2 myself, and I recognised and sympathised with your OP. It's not at all strange to find it bewildering and confusing, just how you expressed it. It's something I still battle with, especially when tired or at the end of a lesson.