just think Used to introduce an idea or possibility that one thinks is particularly exciting, promising, or desirable.

From Longman Dictionary of Contemporary English (just) think used to ask someone to imagine or consider something Just think – we could be millionaires! (just) think of It would be lovely, but think of the expense! just think what/how etc Just think what could have happened. → think Examples from the Corpus just think what/how etc • And just think what the totals would be if you could make a Future Wager in California.

"just to think" is correct and usable in written English. It is typically used to emphasize the idea that something is difficult, strange, or unexpected. For example: "It's hard to believe, just to think that a year ago we were in college.".