how to write it in english?

You are cheating yourself.

You are kidding yourself.

You are deceiving yourself.

Self-deception is a noun, and you wouldn't say "You are self-deception." (譯作"你是自欺) This sentence does not make any sense.

To express what you really mean, try:

You are deceiving yourself.

OR

You are indulging (沉迷在) / engaging (處於) in self-deception (自欺之中).

自己欺騙自己= self-deception.
Also called self-deceit (noun)
圖片參考：http://cache.lexico.com/dictionary/graphics/luna/thinsp.png
自欺
你自己欺騙自己
You are self-deception.

In my opinion, hope this can help you!

self-deceit

or you can say, "you are just kidding yourself"

Yes, you are right.