Sertraline - How long is it possible for the side effects to last with 25 mg a day. Just ending my?

... 3rd week and still lightheaded and having rough moments. I am 86, female. The lightheadedness causes me to feel anxious. My bowel movements are quite soft. I have no appetite. I eat healthy meals, small portions, and take my vitamin supplements. Do not sleep well. My doctor has left it up to me as far as starting on 50 mg a day but right now, I am feeling depressed and fear that I may suffer these side effects always. I am agoraphobic but have handled it well since I was 25 but last...