You generally don't write/sing a love song if you are sure it's going well. You really don't need it. You're already in heaven. That's why people are picking up the sadness, especially from Adele. But, it's strong in the song too. I love the phrase "highway of regret". And traditionally, it is a bad sign, if she/he is not sure yet.
A great song sung by a great singer. I hope she sings forever.