I should have thought all men were born my slaves,
And worn my power like lightning in my eyes,
To have destroyed at pleasure when offended.
- But when love held the mirror, the undeceiving glass
Reflected all the weakness of my soul, and made me know
My richest treasure being lost, my honour,
All the remaining spoil could not be worth
The conqueror's care or value.
- Oh how I fell like a long worshipped idol
Discovering all the cheat.
On the subject of the poem, the titular Angellica, I have to wonder if it's an allusion towards the Angelica of the Roland stories, though the places don't quite seem to fit (Roland's love for Angelica was unrequited, not the other way around). Regardless, we don't need biographical details of the titular character to look at the poem.
To summarize the poem, "Before I fell in love, I thought myself powerful and immune, and now that I have fallen in love, I see my true self, my weaknesses, and my self-image, like an idol, falls." Love shows us the weakness inherent in ourselves, that we'd rather not see, and the act of falling in love can totally crumble our self-image. For Behn, she thought herself a queen, wielding supreme power over men, able to dash their hopes "at pleasure when offended." When she fell in love, that image "fell like a long worshipped idol." She had constructed and worshiped a false self, and its fall was deeply painful.
I suspect however that this fall was fortunate, and was liberating in some way. Love is the "undeceiving glass," that mirror which "reflected all the weakness of [the] soul." She had lost her honor by constructing this false idol, this false self, and love had shown it to her. Whatever plunder she could have gained when she wielded her false power couldn't be worth it, having known love. At its core, this poem is a liberation, albeit painful, from false self image due to the true reflection of self that love provides.
Aphra Behn is very much a poet worth knowing about. While little is known about her in the way of biography, we do know she was a 17th century English dramatist, and sometimes agent of the state in the field of intelligence.