The speaker admits to a long history of dishonesty. "Many many lies" has a tired, almost embarrassed ring, as if the speaker knows exactly how little their word is worth. Then the last line turns. Of everything they have said, "I love you" is the one thing that was true.
What makes the poem work is how plain it is. There are no fancy images, just a confession and a claim. The confession actually helps the claim: by owning up to being a liar, the speaker makes the love feel more believable, though of course a liar saying "trust me this time" is also a risky position to be in. We can't know whether the listener will accept it.
The mood is quiet, vulnerable and a little desperate. The speaker seems to be pleading to be believed on the one point that matters most. The title, a name, suggests the words are meant for a particular person, perhaps Maria herself.