Hi. I hope this correction will be helpful... I have tried to correct each sentences but sometimes I  had to change the order or entirety of the sentence to make it flow... if that makes sense.

I change the tense of your diary to past tense as you are recounting what you have done on your trip.

I also used synonyms to avoid repetitions.

Hope you have more corrections to choose from...

Take care 

I'm wiritng about the continuation of diary that taking a trip to America on Mar.6th.

I am quite confused at this point... you shouldnt need to introduced your next diary's entry???

I had been western United States for 10 days in total.

I stayed (or /went/was) in the (west coast of the) United States for a total of 10 days.

And I rented a car for rental for 6 of 10 days, so I had been driving for those days, in the Grand Circle.

I hired a car for 6 days so I could do the Grand Circle road trip.

It's consisted of the Grand Cyanyon, the Monument vally, the Antelope Cyanyon and so on.

It consisted of going to the Grand Canyon, the Monument Valley, the Antelope Canyon and so on.

Although these places had been exciting me while visiting, in the end of car trip, it made me so sad BECAUSE I got a serious problem with car.

Although I was excited during my road trip, I was left disappointed at the end of it because of a major issue with the car.

Well, it was a flat tire.

Well... it was a flat tire.

I didn't know how to get the problem, but it happened suddenly.

It was so sudden that I don't know how it happened.

I'm guessing that because of driving at high speed(ofcourse, at permittted speed) for a long time, so I should had had take some brake at my discretion.

It might be because of driving for a long time, at high speed  (of course, at permittted speed only). Perhaps, I should have taken some breaks. But it is just a guess!

Anyway, as soon as I found I got the flat tire, I looked for a exit. And then I could went out of hwy.

Anyway... I took the nearest exit of the highway as soon as I had realised that my car's tire was punctured.

If I couldn't had found the exit, I would caused an serious accident.

I think I could have caused a serious accident, if I didn't find it (=the exit).

The diary is to be continued.

(To be continued...)