What historical context can help me understand the line "So have I also, often in wretchedness" from "The Wanderer"?

You are right to seek out the historical context for this ancient, Anglo Saxon poem in order to understand the meaning of that line. First, let us put the line in context and then talk about its historical significance. The following is from the Old English translation from my own textbook:

So I,

often wretched and sorrowful,

bereft of my homeland,

far from noble kinsmen,

have had to bind in fetters my inmost thoughts

We learn from the first stanza that the speaker feels lonely, "wretched and sorrowful" because he seems destined to wander the open ocean on a sea voyage. During Anglo-Saxon times, sea voyages were common, long, and treacherous. There were no modern conveniences. Sails and stars were the useful implements. Sailors were at the mercy of the weather. Obviously, from the lines in the quote above, the wanderer is without his "noble kinsmen"—his friends and family. The wait to see family and friends again will most likely be "for a long time," as the speaker tells us in the poem.  

Another important part of the historical context (although not contained in the quote you mention) is the Anglo-Saxon transition from paganism to Christianity. Note that even in your quotation, old paganism does not provide enough hope for the wanderer. The only thing that does is hinted at in the first and the last lines of the poem. The first lines mention "the mercy of the Lord" while the last lines reveal the following:

It is better for the one that seeks mercy, consolation from the father in the heavens where, for us, all permanence rests.

As a last mention of historical context, we should note that the Anglo-Saxon life—especially an Anglo-Saxon sailor's—is one of uncertainty. Disease, weather, famine, and death are all realities for the Anglo Saxon. Mercy from God is all Anglo-Saxon wanderers can hope for.