Brendawn's line by line analysis is excellent. I'd add a slightly different interpretation.
I find more hope and renewal than disappointment and bitterness here. My reading hinges on several points. First, he says he has found all Love, everything he thought was lost or dead to him, in the bosom of the person he writes this poem to. He has found a lover with a capital "L", and his connection with the person has allowed him to reconnect with all of the love he has felt in the past. He says he finds (in her) that which he "[lacked] and have supposed dead"; because he lacked those feelings, he thought they were dead to him or gone forever, but that does not mean that they are gone now. He finds that although he cried as if he had lost all love, it now appears that it was not gone, but that it was merely removed and hidden within her(4-8). The next part is the section that makes many read this poem as bitter--his metaphor is that she is a grave, a keeper of dead love, all of his dead loves. Here's where my reading diverges:
He does not say that she kills love or destroys it, but that it lives within her. Consequently, though he uses a metaphor of death, death functions to remind us of life. He thought he could never love again; he thought all love he felt in the past was dead and buried. However, if they were buried, they were buried within her. Now whatever love he would have given to others he can only give to her, and he gives her his all.