Hello Bahareh,
I wouldn't place too much emphasis on motivation. Discipline is a better approach. We all know that motivation comes and goes. Discipline, however, will keep you on track. Establish a daily routine for whatever language you are learning. Maybe it's having at least one conversation, learning and practicing vocabulary, or watching something in English. Stick to your routine every day. Developing this kind of discipline will help you on those days when motivation is low.
Take care,
Chase