Love, in this poem, is compared to something that seems pointless: looking up at the stars when clouds are hiding them. The speaker says they love someone "in the way that" people still stargaze on cloudy nights, which suggests a love that keeps going even when there's nothing visible to reward it.
The comparison can be read a few ways. Maybe the love is unreturned, or the person is out of reach, like stars behind cloud. Maybe the speaker just can't see where the feeling is going. Either way, they keep looking up. There's stubbornness in that, and also hope, since the stars are still there even if they can't be seen.
The mood is quiet and wistful, a little sad but not bitter. The title, "heartache," confirms the pain, yet the image is gentle, almost tender. The short lines and lack of fuss make the feeling seem honest and unguarded, as if the speaker is admitting something softly rather than making a big declaration.