Goodbye,_my teacher (Maybe have there are many mistakes ._Please help me.)

Maybe it's [in formal writing: it is] time to say goodbye to my teacher, but I don't [in formal writing: do not] want to say it now. I like my teacher, not just only is he is a good teacher,_but also because since I met him,_I know that my world is not only filled with sadness ._Happiness still exists. But today all classes are over, and I must say goodbye to him. i'm  I felt sad ._I said nothing to him and left school directly._Why? because I don't did not want to cry in front of him._I know that everything has an ending  .

N.B.
1)  A space is needed AFTER (NOT before) every punctuation mark (except after an open bracket or an open quotation mark).
2)  The subject pronoun of first person singular -- I -- is ALWAYS capitalized.

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Good week.  /  祝 今週愉快。