It is impossible to give a line-by-line explanation because the lines do not express complete thoughts, but I can break down the thoughts in the poem.
The first four lines of the poem work as a unit and mean the following: when I die, and nothing matters anymore ("is vain"), what will comfort ("assuage" means to comfort) me for the pain I cannot forget, and what will teach those of us who can't forget to forget? The speaker in these lines is feeling quite sad and questioning whether even death can take away his painful feelings and memories.
In the next four lines, the speaker asks another question: will he be able to find peace after death? He asks if the peace he desires will be sunk, lost in an unreachable place deep in a stream—or will his soul walk through a green meadow ("plain"), where he will come across a fountain of water that he can lean into and there pull up ("cull") the protection ("amulet") of peace? In these lines, the speaker expresses his uncertainty about obtaining peace of mind after death. He wonders if such peace will still be lost, as in life, but also expresses hope that there might be a fountain in the afterlife where he can find peace. He compares peace to an amulet, a charm or necklace that can protect us against danger or despair.
In the last sestet or six lines, the speaker calls his soul pale ("wan") and describes the "golden air" of paradise. He imagines being amid Biblical petals blowing around as he looks ("peers") for the gift of grace that he has not yet received. That gift should not be anything else but one written "spell" or enchantment. That enchanting gift would be the hope of finding his beloved's name in heaven too. He says in the last line that he asks for nothing else but the word that his beloved is there. In other words, he is not asking even so much as to be reunited with the beloved but to know that this person is also in heaven. This will give him peace.