The speaker has evidently lived a long, hard life and has had to suffer numerous unanticipated accidents, disappointments, misfortunes, losses, and possibly episodes of serious illness. A paraphrase should be something like this:
"Under all the unexpected and unavoidable misfortunes, hardships, disappointments, and accidents which a long lifetime holds in store for every mortal" (My head is bloody but unbowed, etc.)
"Bludgeonings" is a pretty strong word. It suggests being beaten over the head with a club or other blunt instrument. However, it is probablly more metaphorical than literal. The line you quote suggests that nobody can anticipate what will happen to him or her in the future, but some bad luck is inevitable. It is largely a matter of "chance." Looking back over a long lifetime, it could seem that the sheer accumulation of all the painful, unexpected, and unmerited things that occurred were like "bludgeonings."
Further suggestions for paraphrasing this line: 'In spite of the blows that fate rains down on me', or, 'under the hardships life brings'.
The poem has a calm aura about it; the speaker seems quite assured of his continuing endurance against all manner of adversity. Henley himself struggled with a terrible, if common illness of his time: tuberculosis. This resulted in him losing his left leg from an early age, and there were fears for a time that he might lose the other leg as well. (He died at a fairly young age from the disease.) This kind of experience was surely what he had in mind when he wrote this poem, although overall he did prosper as a writer and critic and was friends with leading writers and artists of his day. One of his best-known friendships was with Robert Louis Stevenson who apparently modelled the figure of the one-legged, charismatic pirate Long John Silver in Treasure Island on Henley.
'Invictus' remains the most well-known and popular of Henley's writings. It is not hard to see why. It is short, compact, easily memorised, and its positive message of the invincibility of the human spirit, come what may, continues to inspire.