@Fanatic_Francis I don't think he failed. I believe he already knew that Frieren reciprocated his feelings, but he understood it would take decades for her to finally realize them. Maybe that's why he never confessed to her. That's my way of thinking, at least.
When the sky has cleared, and the storm has passed
We'll walk arm in arm, down our promised path
We'll watch the sun come up, from its bed of black
We'll enter Eden's Garden, and never look back