Am I an Old Maid?

This is the phrase you need.  Despite Tor's comment, we do have a phrase for an older, unmarried (unmarriable?) woman.

Yesterday, my good friend from senior high school sent me a message, which said that she is going to be married. Wow, that really made me mad, but only for a few minutes. I soon calmed down and decided it was nothing but another married person among many.

Am I an old maid since I am only 25 years old (at which I think I am still not older enough to get married)? Yet so many friends at my age have got married or are going to be married.

What can I do? Should I look at this thing in an optimistic way? I have no idea.

Good work. :)

Am I a left girl?

Am I a spinster?  / Am I a "leftover" girl?

[There is no real direct translation for this in English, since in the West, being 25 and unmarried is completely normal.]

Yesterday, my good friend in senior high school sent me a message,it said, she is going to be married. 

Yesterday, my good friend in senior high school sent me a message, saying she was going to get married.

wow, that really made me mad,but only for a few minutes.

Wow.  That really made me jealous for a few minutes.

[Getting mad would be a very strange reaction for Westerners.  Friends getting married is a very, very happy thing.  The best substitute I can think of is "jealous" and even that is not a great thing to say.]

I calmed down very soon and thought it was nothing but another married people among many.

I quickly calmed down, and thought to myself, many people are getting married.

Am i left girl

I am a "leftover" girl.

since i am only 25 years old at which I still think I am not older engough to get married.

I am only 25 years old, but I still think I am not old enough to get married.

But so many friends at may age have got married or are going to be married.

However, many of my friends have already gotten married or are going to be married soon.

What can I do? should I look into this thing in a optimistic way, I have no idea.

What can I do?  Maybe I should look at this optimistically.  I have no idea.

[From a Western perspective, I think you should not be worried at all that you aren't married at 25!  There's a good chance you might think this is a blessing later.  Don't live your life based on what someone else thinks you should do.]