“I think I blew it.”
You had a job interview. Your roommate asks how it went. You think that you did a really bad job in the interview, so you say this.
I think I blew it.
In spoken English, you can say "I think ___" before the idea that you're thinking.
I think I'm finished.
I think she's coming.
In formal writing or when you're speaking carefully, you should use "I think that ___" instead:
I think that we need to do a lot more testing before we release it to the public.
When someone messed up or wasted an opportunity, you can say that they "blew it".
Here are some examples:
I could have talked to her, but I blew it.
I was so close to winning, and I blew it!
You can also tell someone:
This is your last chance. Don't blow it!
This is a casual expression, but it's not rude. You can use it around most people.