Four short lines, and a lonely picture comes out of them. The speaker is "on a mission to drink," which sounds purposeful, even a little grand, but the next line undercuts it: "I forget many things." Drinking and forgetting seem to go together here, and the "mission" starts to look like a way of getting away from something rather than toward it.
The second pair of lines is quieter and stranger. "Dancing with myself" suggests someone with no partner, moving alone, maybe lightly drunk, maybe a bit sad about it. Then "after a little talk together" adds a gentle, almost funny touch: the speaker has apparently had a chat with themselves first. The title, "Missionary," plays against all this, since a missionary has a cause and other people to reach, while this speaker seems to be reaching only inward. The mood is wry, solitary, and a little tender, with loneliness handled lightly rather than dramatically.