Suggest Remedy For Chronic Constipation

I am an active and healthy 65 year old female, but I struggle with constant constipation. I do not go on my own. I always have to take a laxative and I take one twice a week to go to bathroom. I have been taking Ducolax and also tried Phillips Milk of Mag....but both always make me nauseated and feel bad for the days I take them. What is this lactulose? Can you purchase over counter? How often and how much would I take. I do take a probiotic daily as well. I just get so tired of fighting this and knowing on the days the laxative is going to work I will feel terrible...every week...2 days a week. What can I do?