Believing and Receiving

Matthew 21 (King James Version)
21 Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.
22 And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.

So there we have a very early version of believing and receiving.
Of course we are told over and over just why this doesn't work.
There's even a famous prayer (not in the Bible) that says

"God, give us grace to accept with serenity the things that cannot be changed, courage to change the things that should be changed, and the wisdom to distinguish the one from the other."
-Reinhold Niebuhr
Of course a modified version of this prayer is used by Alcoholics Anonymous.

But doesn't this prayer go against what Jesus said in the Bible?

"God, give us grace to accept with serenity the things that cannot be changed"
"And all things, whatsoever ye shall ask in prayer, believing, ye shall receive"

So why is the "accept the things I cannot change" drilled into peoples heads?
Is it easier than saying that you can change them?
Why is it that give up and surrender seems easier than stand and fight?

And actually when you look at it, there is no fight other than with yourself.