I am in the zoloft 50 mg since 2 months for anxiety disorders and...

I am in the zoloft 50 mg since 2 months for anxiety disorders and hence depression. Initially it gave me abnormal side effects but after two or three weeks it slowed down now there is very low side effects. I used to feel that i am going to die or my heart is going to stop work in the time of side effects. it worked me initially and i felt good for few days. I can work, i can live happily i used to think so but now a days my previous condition is trying to return back and i feel anxiety again but i am still taking the medicine. can any one suggest me why it returned back? is it due to the low dose or what? dipak