To say "I miss you so much" in French, say: "Tu me manques tellement" or "Tu me manques trop." The latter is grammatically incorrect but more widely used colloquially. They can be pronounced phonetically like this: "Too muh monk (as in honk, not monk) tell-mon" or "Too muh monk tro."
If you are saying goodbye to someone really special to you and want to speak in French there are also some other useful phrases you may think of using. These include the following:
"Je t'adore." (I love you - between friends.)
"Je t'aime" (I love you - romantically.)
"Tu vas me manquer" (I'm going to miss you.)
"A bientôt." (I will see you soon or I will see you later.)
"Je ne peux pas attendre de te revoir." ( I cannot wait to see you again.)
"Je vais toujours penser toi." (I will always be thinking of you.)
"Nous devons rester en contact." (We need to stay in contact.)
Of course, there are hundreds of things you may wish to say to someone when saying goodbye. If you know someone who speaks fluent French, ask him or her to help teach you a few phrases. Alternatively, there are a huge number of websites you could visit that offer free translation services. All you need to do is type in the text in English you want to see translated into French and then click on the 'translate' button. It should be noted though that not all these online translation services are 100% grammatically accurate all the time.