“th” sounds require precise tongue placement at the front of the mouth, and that can increase tension, especially when you’re anticipating a block. For “th,” thinking more about reducing overall articulatory tension and using gentle airflow can help — not forcing the tongue, but saying the sound with light contact. Also, pause before the difficult word. It may be worth working on stuttering openly and reducing the struggle around it like tension, or forcing. This takes away the pressure to be perfectly fluent, which can ironically reduce severity over time.