Assalaamu alaykum. I want to do a certain optional worship daily, but I find it difficult to do that, so to motivate myself, I reward myself. For example, I say to myself, if I do this good deed today, then I will eat such-and-such delicious food in such-and-such restaurant. Is that okay?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger. 

There is no religious impediment that a person rewards himself with something lawful when he does a good deed to motivate himself to continue to do it. On the other hand, it is also allowable for the person to punish and discipline himself by depriving himself of something lawful if he commits a sin or Sharee'ah violations. It has been reported that some scholars did so. The honorable Companion Ka‘b ibn Maalik offered part of his wealth in charity when Allaah accepted his repentance (after the Battle of Tabook).

Allaah knows best.