I love you not only for who you are but for who I am when I am with you
"Cuando estoy contigo" means "when I am with you" in Spanish. It is used to express being together with someone.
Estoy feliz cuando estoy contigo.
'Cuando vuelva, hablaré contigo.'
Translation: Estoy triste cuando no estoy contigo. You could also say "Me entristezco cuando no estoy contigo" (I sadden when I am not with you), but this is more literary, although the emotion comes off more strongly. Just as a note to your translation: "con tú" does not exist in Spanish and always becomes "contigo". (The same with "con mí" becoming "conmigo". "Consigo" has a set of unique rules which is why "con él", "con ella", and "con Ud." do not automatically become "consigo".)
Espero hablar contigo(informal)/con usted (formal) de vez en cuando.
"Cuando" translates to "when" in English.
It means "I miss you when I don't talk with you."
cuando
You can say in Spanish, 'Nuestra relación se acabó' o solamente 'terminamos'. This last word is the same term as 'we broke up'. You can say 'se acabó' the same if you say 'it's over'.
es cuando
"Hasta cuando" in Spanish means "until when" in English. It is a phrase used to inquire about the duration of something or to express concern about how long a situation will last.
Cuando Cuando