"Ever" occurs four times in this poem, and in such a short poem, that kind of repetition really stands out, especially when three of those repetitions come in the first three lines. Our clever speaker uses the word in two different but somewhat related senses. In the first three lines, the word means something like "at any time." In the last line of the poem, the word means "forever," as in eternally. Both uses of the word suggest that the speaker of the poem is obsessed with thinking about long periods of time, or perhaps all of time itself. She's a romantic, after all, and what's a romance if it doesn't last for all eternity?
Line 1: The speaker says that, if any two people were ever one, it is she and her husband. Basically, she's comparing their love to that of every couple that's ever loved in the world. She seems pretty darn sure of herself.
Line 2: If ever a man were loved by his wife, it is sure that the speaker's husband is. This line continues the idea set up in line one.
Lines 3-4: The speaker tells other women to try to match her when it comes to being happy with one's man. The repetition of the words "if ever" in successive lines (1-3) is called anaphora, and it's a handy device poets like to whip out every once and a while to add emphasis to their points. In this case, it emphasizes – no surprise here – the love between the speaker and her husband.
Line 12: The speaker says that she and her husband should "persevere in love" so that they "may live ever" (i.e., forever). There's really nothing more romantic than that, right? Sigh.