Insecure meaning

not firm or fixed; liable to give way or break.

(of a person) uncertain or anxious about oneself; not confident.

subject to fears, doubts.

not self-confident or assured: an insecure person

Lacking confidence and doubting their own abilities

lacking self-confidence or lacking security

Similar words 

unconfident

uncertain

unsure

doubtful

self-doubting

hesitant

unassertive

retiring

shrinking

shy

timid

timorous