Here, a speaker looks back on a first love that didn't last. Early on, the memories are sweet and simple: a first kiss, a first valentine, and a boy who shared his dreams. The speaker assumed she would be part of those dreams, and she still calls him her first love, with a word that suggests it will never fade ("will always be").
The tone shifts in the last two stanzas. As the two grew up, he stopped being hers, and he went on to share his life with someone else. The ending is the most painful part. His dreams came true, but hers didn't, because every one of them included him. The speaker seems to be left stuck in the past, still loyal to a future that never happened.
The mood is wistful and quietly sad, with a plain, direct voice. There's no anger here, just a lingering ache. The title's question marks, "First???", also suggest she may be wondering whether being first meant anything at all.