I can understand the challenges of analyzing a poem. There is no one correct way of doing this. If we start with this premise, then you can engage the poem on many different levels, which may produces some interesting findings. Here are some approaches that you may desire to take. First, you can do a little research about the historical context in which the poem was written. This may be a window into that society. So, you can take a historical perspective. Second, you may want to study reception. In other words, you may want to explore what others have said about this poem. Keep in mind that often times a good poem tells you more about an reader than the poet. Finally, you can focus on the literary qualities or mechanics of the poem. Of course, there are many other approaches, but these should get you started.
Poetry is not that subjective and the subjectivity deals with slight variations that make the poem meaningful to different people in different ways. It doesn't mean that you can't interpret poetry, and it doesn't mean that you can interpret it to mean anything you want. And the fact that poetry is emotional certainly has nothing to do with whether or not it can be interpreted. You don't have to have felt the exact same emotions a poet presents in order to understand those emotions. That's one important role of literature: to introduce readers to experiences they haven't had.
That said, here's the poem:
Women have loved before as I love now;
At least, in lively chronicles of the past—
Of Irish waters by a Cornish prow
Or Trojan waters by a Spartan mast
Much to their cost invaded—here and there,
Hunting the amorous line, skimming the rest,
I find some woman bearing as I bear
Love like a burning city in the breast.
I think however that of all alive
I only in such utter, ancient way
Do suffer love; in me alone survive
The unregenerate passions of a day
When treacherous queens, with death upon the tread,
Heedless and willful, took their knights to bed.
The speaker is in the process of reading tales of women who have loved like she loves. As she reads, she is
Hunting [for] the amorous line, skimming the rest,...
Looking for the romantic parts, she finds in literature the same feelings she feels, and thus knows that, at least in literature, women once loved as she loves. The speaker feels an all-encompassing love and feels that no one else quite loves the way she does--who doesn't feel that when he/she is really in love? This doesn't mean she's arrogant or full of herself. It means she's in love.
The speaker revels in a woman's right to love in the fashion of men, passionately and fully, not worrying about the consequences, as men have loved and been made heroes because of it throughout the centuries. The speaker loves like a queen who has the power to do whatever she wants loves. The speaker suggests that all women should love this way, as, at least stereotypically, men have been doing for thousands of years.
Millay is comparing herself to famous women in the past whose love changed history. "The Trojan waters by Spartan mast" refers to Helen of Troy and the Trojan war that resulted over two men's love for her. I'm not sure who "of Irish waters by Cornish prow" refers to. "The cost invaded" refers to the effect on history these women had, especially with the death of soldiers and the fall of kingdoms. These women sought love and caused consequences like the burning of a city (Troy). Millay thinks she is the only one now who feels this kind of love and is a survivor of those famous loves of the past. These women did not regret what their love caused and she does not either in "the unregenerate passions of a day". The queens she refers to may be Queen Guinevere of King Arthur fame, Lady Jane of England, Ann Boleyn, or even Queen Elizabeth the first. This is my best educated guess at the poem. She sounds pretty full of herself to be comparing her love to those of history, don't you think? Don't worry about not being able to interpret poetry. It is very emotional and subjective. Unless you have the same feelings as the poet, it may not make sense to you or other readers.