General synonyms:
* Anxious
* Worried
* Concerned
* Fearful
* Nervous
* Uneasy
* Scared
* Troubled
* Disquieted
More specific synonyms:
* Dreadful: Feeling a sense of dread or fear.
* Timorous: Showing or suffering from timidity; easily frightened.
* Suspicious: Inclined to believe that people are dishonest or have bad intentions.
* Hesitant: Unwilling to act or make a decision.
* Doubtful: Not certain or confident.
The best synonym to use will depend on the specific context and the nuance you want to convey.