Yesterday, the food was absolutely terrible. However, today the chef and I are best friends! Because of this wonderful change, I am very happy right now.

was: "Yesterday" tells us this is in the past, and "food" is a singular uncountable noun, so we use "was".

are: "Today" tells us this is in the present. "The chef and I" is a plural subject (we), so we use "are".

am: "Right now" tells us this is in the present. The subject is "I", which always takes "am" in the present tense.