"I miss you" - this is an assertion independent of time. "I am missing you" - presently and continuing to miss you. "I missed you" - previously I was missing you (presumably said when you're no longer apart, so it's no longer in the present). We also say "I missed you" when you failed to meet. "Sorry I missed you last night". This is a different meaning of miss, "failed to intersect with", and not really what you're asking.

I read it as: "I miss you" (...because you're not here now) "I missed you" (...but you are here now)

"I missed you" its is also used in terms fo lossing someone or somthing. for example, I missed my target, I missed my train, same is the case here, I missed you, mean I have lost you by doing some misstake or through some missunderstanding. And when your gf sends you a message, "I missed you", it means that she was just missing you a few minuts or a few moments befor. For example I am busy in doing some work and at the moment I miss my love and after missing him/her i send a message as "My love I missed you, therefor I am sending you the message."

Use the word I miss you when you are seperated. Use the word I missed you when you are together. Check out my site https://bigben.academy/the-use-of-miss-or-missed-all-differences-explained/, will explain to you more :) hope it helps