Onemight be tempted to suggest togetherness or contentment, but the fact that these two other words bear definitions unrelated to each other perfectly displays why lonesome cannot be properly mirrored. It does not mean solitude, nor alone, nor lonely, although lonesome can contain all of those words in itself.
Lonesome means a state of being apart. Of being other. Alone-some.
He leaned toward her—her heart spun again—and pressed his cheek against hers. His lips didn’t touch her skin, but she felt his breath, hot and uneven, on her face. His fingers splayed on either side of her spine. Her lips were so close to his jaw that she felt his hint of stubble at the end of them. It was mint and memories and the past and the future and she felt as if she’d done this before and already she longed to do it again.