Too vs very - what's the difference?

Very adds emphasis - it's neutral. Too means more than is good or acceptable - it implies a problem.

"The soup is very hot." - compliment or observation; hot is good
"The soup is too hot." - I can't eat it; the heat is a problem

"She's very confident." - positive quality
"She's too confident." - criticism; she has more confidence than is appropriate

A classic pattern:
"It's too heavy to lift." (so heavy that lifting is impossible)
"I'm too tired to go out." (so tired that going out isn't possible)

Using too when you mean very:
"The party was too good!" - technically means it was problematically good, which sounds odd
Say: "The party was really good!" or "so good!"

"Not very" softens. "Not too" often means quite acceptable:
"It's not very warm." - it's cold
"It's not too bad." - it's actually okay (British understatement)