grateful, indebted
忝 centers on feeling a deep sense of unworthiness when receiving kindness or favor: being grateful yet indebted, as if the favor is more than one deserves.
忝 combines 天 (heaven) with 心 (heart), suggesting a heart that feels small before heaven's grace, leading to the idea of humble gratitude and indebtedness.
Heaven 天 above a heart ⺗: imagine receiving a blessing from the sky so great that your heart feels unworthy and deeply grateful.
For テン, picture ten (テン) kind acts from heaven that make your heart feel indebted: ten -> テン, and each act deepens your gratitude.