Figuring out a song’s harmony can be a tricky challenge.
That’s true when transcribing harmony from a song, or even when composing a song yourself! More than once I’ve encountered a student with a strong melody who had harmonized with what was clearly the “wrong” harmonization. They knew it was wrong, or simply couldn’t figure out the right harmony.
The combination of intuition, logic, music literacy, and instinct develops over time; at first, it’s a lot of trial and error.
(As Alice Parker would say, the easiest to harmonize well are the strongest melodies; they tell you clearly what they want.)
Work on the melody, chord by chord, and take the time you need. Next time, you’ll have built the skill a little more. It gets faster.