William Shakespeare's "Sonnet 30" is about the speaker feeling dejected when remembering past woes and losses. For example, in the first two quatrains, he "drowns an eye" remembering "precious friends" who have passed away, and he wastes his time thinking about "old woes." However, his sorrows end when he thinks of his "dear friend" in the couplet, and all of his losses are restored.
Essentially, the speaker of "Sonnet 30" is recalling sad events in his past, and feels the pain over and over again. In the third quatrain, the "sad account" in line 11 is equivalent to an itemized bill requiring payment. This metaphor refers to the speaker's sorrows that never seem to go away but keep plaguing him, just like a bill that has already been paid but continues to demand payment. This means that the speaker must suffer these "fore-bemoaned moans" continuously even though they are in the past and he has already mourned them.
Lucky for him, he has a friend to cheer him up.