To be on an emotional rollercoaster (roller coaster) means to experience swift and sudden changes in one’s mood or feelings, 

similar to the movement of a roller coaster at an amusement park.

For example:

I’m on an emotional roller coaster watching this movie—one minute, I’m laughing, then the next I’m crying!

Before my mood disorder was diagnosed, I felt like 

I was constantly on an emotional roller coaster.

Yeah, you’re definitely on an emotional roller coaster

when you’re traded multiple times in one season.

Her relationship with him was a constant emotional rollercoaster; one day they were in love, the next they were fighting. 

The journey through university is often an emotional roller coaster for students as they navigate new experiences and challenges”. 

Tradução:

Montanha-russa emocional, turbulência emocional, gangorra emocional, ser apanhado num turbilhão de emoções.