It might work best as two sentences since you are saying two things: 1) Journaling increasing positivity, 2) Positivity doesn't happen straight away. "If you keep a journal, you might become more positive. However, this might not happen overnight."

Thank you all!

When I was in high school, an English teacher told me that clauses starting with "but" always follow a comma. I don't know if that's a hard and fast rule of grammar, but it has served me well. :)

i think, you don't need to use a comma there.