I am non assertive type person. Sometimes my bosses and collegues take undue advantages of this and cannot reuse them or assert myself . How I can become assertive in my life.
You say that others take advantage of you, so that tells me some good news: You know what is right and what is not right, what is healthy interpersonal behavior and what is not.
Then comes your big question, “Why do I not stand up for what is right? What attitude of mine makes me willingly participate in unhealthy behavior? Is it fear of others’ disapproval? Fear of making a mistake? Low self-esteem? Not caring enough about anything to take a stand?” Work out the answer for yourself. Then will you know better what to do about it.
To help you work this out, I suggest you experiment: Stand up for something you are sure is right. Just do it. Create a personal boundary with regard to some situation or some person, and hold to it. See what happens. If the boundary turns out to have been appropriate, then people will begin to respect you more for having it, and will take advantage of you less. And if it later turns out that your boundary was inappropriate, then admit it, apologize as needed, learn from the episode, and continue with your experiment. (It’s best to experiment with boundaries that will not have catastrophic consequences if they turn out to have been unwisely chosen.)
The worst case scenario is to allow the current situation to continue. Begin to stand up for yourself, experiment, learn, and grow — from your mistakes as well as from your successes.
I wish you all the best.