When we two parted In silence and tears, Half broken-hearted To sever for years, (1-4)
This is no ordinary goodbye. There is definitely a lot of love here, otherwise they wouldn't be broken-hearted. Right? The exact nature of this love isn't clear. Is it romantic love? Filial love? Platonic love? Who knows?
Pale grew thy cheek and cold, Colder thy kiss; (5-6)
Well this is what the death of love looks like. A woman who used to be so "warm" is now "cold" and gives the speaker an even "colder" kiss. Sounds she's just going through the motions at this point. Poor fella.
Thy vows are all broken, And light is thy fame; (13-14)
"Vows" make us think of marriage—the ultimate sign of love between two people. As in the poem's first stanza, we are given here an image of love's destruction: a broken promise or relationship.
Long, long shall I rue thee, Too deeply to tell. (23-24)
Yeah, he says he'll "rue" her really deeply, but that's only because he's loved her so deeply. Right? Right? You can't feel this sad unless you're really, really in love, or obsessed. That's what we think, at least.
In secret we met— (25)
Okay there was definitely something inappropriate about this love affair. Seriously, who meets in secret except people who aren't supposed to be meeting? It sounds like this love affair crossed some serious lines.
In silence I grieve, That thy heart could forget, Thy spirit deceive. (26-28)
Here it sounds like we have an instance of the old "I love you more than you love me" thing. The speaker implies that he still loves the woman, but apparently her "heart" (the seat of emotions like love) has forgotten all about him. Sheesh—forget her then.