In "Sonnet 43," the phrase "I love thee" is repeated nine times in the poem. This is called anaphora: the repetition of a word or phrase. This is often used in poetry but also in political rhetoric with phrases such as, "I believe in an America that . . ." Such a device in political speaking is to emphasize the speaker's love and loyalty to America. The effect of the repetition in the poem is to emphasize how much the speaker is in love. Therefore, the tone of this poem is uplifting and illustrates indescribably happiness and love.
The imagery the speaker compares her love to is abstract and therefore difficult to quantify. The effect is that her love is so great that it defies description or comparison with ordinary, worldly things. In lines 2-4, her love is as deep as the soul can reach; as far as the ends of the world (Being) and even as far as God ("ideal Grace"). This is a love that is Earthly but at the level of the spiritual. Those lines are tempered with the next in which she loves him (Robert Barrett Browning) at every single moment, no matter how trivial, of the day and night ("by sun and candle-light").
In lines 6-7, she uses similes (comparisons using "like" or "as"):
I love thee freely, as men strive for Right;
I love thee purely, as they turn from Praise.
Here she compares her love to other virtuous behaviors. Again, these are not concrete images or things. They are abstract feelings and beliefs. She loves as much as free men strive for justice and as much as pure men do not need praise. So, this is a love that is virtuous and not proud. In the following lines, the speaker loves him as much as the passion she's given to all the grief in her life and she loves as much as she was trusting when she was an innocent, dependent child.
In the final lines, she loves so much that her faith of love in life and death is renewed. Thus, her love of God is also renewed. The Earthly and Spiritual are united here. Stylistically, life and death/afterlife are linked with the rhyme of lines 12 and 14: breath and death. Usually, rhyme is described as a formal or aesthetic device but it often, and this is one such case, is used thematically to link ideas.