Answered for language learning in general. No specific language was set for this question, so the findings are drawn from general second-language research rather than one language’s literature.
The gap you're noticing — knowing a rule but not applying it fast enough in speech — is a well-known distinction in language learning between "explicit" (consciously known) and "automatic" (instantly usable) knowledge. The evidence suggests the fix is heavy, repeated speaking practice on your specific error patterns with real-time feedback, ideally in concentrated bursts, rather than more grammar study alone.
What you're describing — knowing something is wrong only when you have time to think — is a well-documented gap between 'explicit' knowledge (rules you consciously know) and 'implicit' or automatic knowledge (what comes out instantly when you speak). Research distinguishes these as genuinely separate systems, and having one doesn't automatically give you the other.4,5,7
In plain terms
Multiple studies describe a shift from 'controlled', effortful use of grammar to 'automatic' use, and this shift happens mainly through repeated practice of the same structures in meaningful, communicative use — not just by learning more rules.2,5,7,10,11
In plain terms
Example
In one study, ESL students who repeated the same short speech three times became measurably more fluent, and unlike students who talked about a new topic each time, they kept those fluency gains afterward. Repeating content — not just talking more in general — seems to be what drives the improvement.10
Example
A study comparing intensive (concentrated) versus spread-out language courses found that intensive practice gave an edge in several skill areas, including speaking, but mainly for intermediate learners — advanced learners didn't benefit as much from intensity, possibly because they need to fine-tune existing knowledge rather than build new automatic skills.2
A study of language learners' speaking errors found the mistakes came from several combined sources: gaps in grammar/vocabulary/pronunciation knowledge, low confidence and anxiety about speaking, and simply not having practiced enough. Shyness and fear of being laughed at were specifically named as barriers stopping students from getting the practice they needed.6,9
A study using automated, detailed feedback on pronunciation errors during peer practice sessions found it improved learning performance, motivation, and learners' sense of control over their own learning, compared to standard peer feedback alone.8
Research on learning stages proposes that learners move through phases from basic knowledge to 'conscious expression' (like your situation — correct only when thinking it through) and finally to automatic, fluent use, and that sustained motivation is what carries learners through these later stages, not just initial knowledge.12
One classroom study found that explicitly teaching students strategies for noticing, monitoring, and practicing a grammar structure (not just teaching the rule) helped weaker learners apply that knowledge better and even transfer it to a related but new grammar structure — though results were still imperfect, suggesting this kind of training speeds progress rather than instantly fixing errors.1
A study of university EFL students found that heavy reliance on AI translation tools boosted vocabulary learning and reading/listening comprehension, but had no measurable effect on speaking skill — and the researchers worried that over-reliance on such tools might actually get in the way of deeper language mastery needed for speaking.3