The speaker is stuck in a one-sided situation: they still think about someone constantly, but they suspect that person has stopped thinking about them. The opening wish, to be "still in the back of your mind / like you are mine," sets up the imbalance right away. Their own mind is full of this person, while their place in the other person's thoughts seems to have faded.
The calling is the sore point. The speaker imagines that a call from them should feel like "temptation," something hard to resist. Instead, the other person just keeps pressing decline, which is a small, repeated act of rejection. The word "keep" suggests this has happened more than once.
The mood is quiet and hurt, with a hint of longing and self-doubt. The lines are short and plain, which makes the pain feel honest and unpolished, like a thought typed late at night.