Motivation is a drive that directs our behavior and leads us to act in a way that will satisfy certain needs, desires or goals.
Our motivations can be either internal or external in origin. More specifically, intrinsic motivation is observed when a person undertakes an activity aiming at a personal benefit and does not require any external reward (e.g. a hobby). Extrinsic motivation is observed when someone behaves in a certain way for reasons that are subjected to external control such as to obtain money or to compromise some compulsion. Extrinsic motivation can come from parents, boss, co-workers (e.g. promotion).
However, regardless of the origin of our motivations, it is certain that, to a very significant extent, they influence the achievement (or not) of our goals.
When you feel that your strength is decreasing, recall all those reasons that motivated you to begin the effort.
Set realistic goals. By setting small, achievable goals, you are more likely to experience satisfaction and feel self-efficacy. Doing so will give you more energy to continue and finally reach the desired result.
Reward yourself every time you take a step towards your ultimate goal. It is very important to recognize your progress and focus on it.
Even if something doesn't turn out as planned, identify what your experience has to "teach" you and continue with this knowledge as an ally on your way to conquering your goals.