'I miss you' suggests that you haven't met the person you're missing just yet, and you're longing to meet him/her. 'I miss you' (you're missing him/her right now as well) However, 'I missed you' suggests that you've met the person you were longing to meet, and upon your meeting, you might have said 'I missed you' (I used to miss you, and now that I've met you, I don't need to miss you anymore as you're with me)
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