The speaker wonders, on certain days, whether they will ever stop sleeping in corners. The image is small and ordinary, but it says a lot. Someone who presses into a corner, with walls guarding the back and sides, is arranging even sleep so that nothing can come at them unseen. The only way to reach them is "head-on," where they can see it coming.
The poem never says why the speaker sleeps this way. Judging by the title, "My Side of the Bed," it may be about sharing a bed, or about a bed that was once shared, though the poem leaves that open. What comes through is a habit of staying on guard that has outlasted whatever caused it.
The mood is quiet, tired, and a little wary. "Some days I wonder" sounds honest rather than dramatic. The short last line, "head-on," lands hard, leaving the speaker's caution sitting there plainly.