Is there a word/expression that would correspond to something like "self-distance"?
Solution 1:
You should consider words such as modesty and humility. These are both seen as being positive traits but could be taken to an extreme where a person doesn't take appropriate credit for their accomplishments or abilities.
Another word to consider would be objectivity. This captures the sense of lack of egotism, but it isn't necessarily understood to be reflexive.
Solution 2:
Self-aware is commonly used to describe a healthy objectivity toward oneself, though the formal definition is more limited.
Self-effacing captures the idea of modesty, potentially to a fault.
I don't know of an English word that combines both concepts in the way your Swedish term does.
Solution 3:
self-possession
Merriam-Webster: control of one's emotions or reactions especially when under stress : presence of mind, composure
Oxford: the state or feeling of being calm, confident, and in control of one’s feelings; composure: an air of self-possession
FreeDictionary: Full command of one's faculties, feelings, and behavior. See Synonyms at confidence: ... Self-possession implies composure arising from control over one's own reactions: "In life courtesy and self-possession . . . are the sensible impressions of the free mind, for both arise . . . from never being swept away, whatever the emotion, into confusion or dullness" (William Butler Yeats).
Although less literal a match than the other answers, I think you may want to consider this term for the purpose you intended, or the person you intended. It seems självdistans emphasizes inner perspective while self-possession is about inner behavior, but they may converge on the same character trait: not being ruled by one's own emotions. After all, which emotion is more remarkable to be free of than self love? I wonder if this distinction may reflect culture, Swedish emphasizing self-image, English self-control. Almost thought versus action.
Whomever deserves this description in your book, I like them already.