"you know" is a short, affectionate thank-you to someone who understands the speaker completely. The speaker admits to being difficult at times: there are mood swings, and a sensitivity that never switches off. Even so, this person has stayed. They have even dealt with the speaker's family, something the speaker says they can't manage themselves, which hints that those relationships may be tricky.
The feeling underneath is relief and deep gratitude. The line "you've passed the test" is playful, but it suggests the speaker may have worried about being too much for anyone. The ending is quiet and simple. When things get hard, the person holds their hand and whispers "i've got you," and that is all the speaker needs. The final line, just "you.", narrows everything down to one person. The mood is warm, tender and secure, helped by the lowercase letters and plain, conversational wording, which make it feel like a private note rather than a polished speech.