It's hard to say for sure. I think both answers could be defended based on the poem. Over the years, I've read a couple of broad overviews about Carol Ann Duffy and her poetry. Some of those articles praise Duffy and her poetry for speaking from a feminist perspective. In that light, I think "Valentine" could be Duffy speaking about love. Duffy is most likely bringing some personal experience into the poem. The uses of "I" and "we" help the reader feel as if the author of the poem is actually speaking of a personal experience.
On the other hand, some critics liken Duffy's poetry to Robert Browning's poetry. Many of his poems are monologues written from the perspective of a disturbed character. Browning's "My Last Duchess" would be a good example of that. It's possible that Duffy's "Valentine" is a similar monologue. Then Duffy would not be the speaker. The speaker would be a character who is in love, and is someone who definitely does not adhere to the traditional romantic love icons of roses and candies. I'm more inclined to support this reading of the poem rather than thinking the speaker is Duffy herself.