“I used to be such a procrastinator!”
You're at a work party and talking with a coworker. You both start to talk about what you were like when you were students. You say this, remembering that you used to wait until the last minute to do homework assignments.
I used to be such a procrastinator!
You use the phrase "used to be ___" to describe a quality that someone had in the past, but doesn't have now. It's useful for describing your life when you were much younger than you are now.
Of course, you can also use "was":
I was such a procrastinator when I was younger.
But "used to be" makes it clear that you're describing something far in the past, not something recent.
The word "such" is similar to "so" except it's used with nouns instead of adjectives:
He's so handsome.
He's such a good-looking man.
I used to be so lazy.
A "procrastinator" is a person who often waits until the last minute to do important things. This word is related to the verb "procrastinate":
Stop procrastinating and get to work!
An important point is that "procrastinating" specifically means waiting until the last minute, not doing something at the last minute. So you can procrastinate for a really long time without actually doing the thing that you were supposed to.