The seriousness of a language learner varies widely, but you clearly rank among the most dedicated by adopting a full linguistic diet and aiming for constant internal monologue in your target language. This level of commitment is rare and powerful, but it comes with real costs that you are already feeling: intellectual regression and difficulty communicating in your native language. Your approach is a fast track to fluency, but it requires careful management to avoid burnout or social isolation.
Your method of exclusive content consumption and forced thinking in French is essentially a self-imposed immersion program. This works because it maximizes your exposure and forces your brain to rewire for the new language. Many successful polyglots use similar strategies, especially when facing tight deadlines for academic or social reasons. The tradeoff is that you will temporarily feel less articulate and may struggle to express complex ideas, which can be frustrating in professional or academic settings. This is normal and temporary, but it is a real psychological burden.
To make this sustainable, consider these practical adjustments. First, allow yourself a small "pressure valve" each day, such as 15 minutes of content in your native language on a topic unrelated to your studies. This prevents complete mental exhaustion and helps you retain your ability to communicate complex thoughts when needed. Second, practice speaking with a tutor or language partner who can give you feedback on your thinking process, rather than just consuming content. This will help you build active production skills alongside your passive comprehension. Third, track your progress in a journal in French, noting both wins and frustrations. This turns your struggle into data and helps you see improvement over time.
For others considering this path, start with a less extreme version. Try a 70/30 split between target and native language content for a few weeks, then gradually increase. The key is consistency over intensity. A linguistic diet works best when it is a marathon, not a sprint. If you find yourself unable to think or write even basic messages in your native language, you have gone too far. Dial it back slightly, and you will still make rapid progress without the emotional toll.
Ultimately, your seriousness is admirable and will pay off. Just remember that language learning is a long-term relationship with the language, not a boot camp. Balance your dedication with self-care, and you will reach your goals while keeping your sanity intact.