The speaker is lamenting the brevity of love. Love, no matter how intense it may be, can so often vanish into thin air. For one reason or another, lovers can fall out of love so very easily. What once seemed destined to last forever can be doomed to destruction in the blink of an eye.

Clearly, the speaker's lover meant a great deal to him. Though the love they shared lasted for only a relatively short piece of time, the speaker cannot stop thinking about all the good times they enjoyed together. That's what he means when he says that "forgetting is so long." It's going to take the speaker a very long time to forget his lover, no matter how often he insists that he no longer loves her.