Why is it that whenever I have a setback instead of finding the motivation to move forward I just become numb and despondent ?

It happens with a lot of people, and actually to some extent with everyone. Sometimes it's just about certain times when nothing seems to get better even after making so many efforts. You can ask for help and there is no surety that it will work for sure but life is about to try to move forward. So keep trying and always spend time with people and do things that make you happy. Don't drown in things that are not working out for you.