The difference between those who succeed and those who fail is a matter of two things - how realistic we set our goals and how hard we are willing to work for them.
A goal that cannot be achieved is not a goal. It is a dream. When we hang on to goals that we know are not achievable all we do is dishearten ourselves. We create a reinforcing pattern of defeat that will do nothing but serve to keep us down.
Worse than not letting us accomplish the goals we have set for ourselves, this sort of downward spiral of failure can taint our other endeavours. This makes it harder to accomplish the things that were formerly easy.
To break this cycle we need to learn to set reasonable goals, and we need to revisit the goals we have to make sure they are realistic.
The other thing we need is the willingness to do the work that those goals require. There is no substitute for effort.