Whatever you think you have to do is simply what you want to do.

[Was Du zu müssen glaubst, ist das, was Du willst.]

Marie von Ebner-Eschenbach (1830-1916) Austrian writer

Aphorisms [Aphorismen], No. 60 (1880) [tr. Scrase/Mieder (1994)]
    (Source)

(Source (German)). Alternate translation:

What you wish to do you are apt to think you ought to do.
[tr. Wister (1883)]