I am a native English speaker, the only language I am fluent in. My mother tongue is Russian, but I only have basic comprehension skills and limited speaking capability. Achieving this goal would require a combination of consistent daily practice, immersion, and developing cognitive skills and tools (memory).

For the purposes of this question, fluency means the B2-C1 levels of CEFR. This means (1) ability to understand the language at a standard spoken rate (TV, radio, podcasts) (2) ability to read texts on nontechnical topics unassisted (newspapers, native dictionaries) (3) ability to fully express myself in spoken and written communication unassisted. It does NOT mean speaking without an accent, reaching native adult/business/technical proficiency, or understanding various dialects (current or historical).