General synonyms:
* Anxious
* Worried
* Concerned
* Uneasy
* Nervous
* Fearful
* Dubious
* Suspicious
* Skeptical
More specific synonyms:
* Dreadful: Feeling a sense of impending doom.
* Timid: Feeling shy or lacking confidence.
* Hesitant: Uncertain or reluctant.
* Trepidatious: Feeling fear and trembling.
* Wary: Showing caution or suspicion.
* Cautious: Showing careful consideration.
The best synonym will depend on the specific context in which you are using the word.