I think (clause)

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.

(someone) blew it

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.