How to Retrain Your Mouth

Pronunciation is a physical skill, not a knowledge problem. You already know how "th" should sound. Your tongue just refuses to go there because it carries 20-plus years of muscle memory from another language.

Use a mirror to watch your mouth. For TH, your tongue tip should be visible between your teeth. For R, your tongue curls back without touching the roof of your mouth. For V, your top teeth touch your bottom lip.