One of the most common questions I'm asked:
"How do you stay so motivated?"
In my eyes, you cannot force yourself to be motivated.
You can't simply click your fingers and poof, there it is!
Motivation comes from, firstly, how you treat your mind and body - nutrition, exercise, sleep, social interaction, mindfulness... your general well-being will affect your motivation.
Secondly, whether or not you are interested, passionate, or receiving a reward from doing something. In this case, if they don't come naturally you need to determine how you can create this passion, interest, or reward.
Alter the way you operate to make it more enjoyable, explore things to determine what you're passionate about, set yourself goals with rewards as a carrot on the end of the stick.
Finally, habit. You can get into the habit of being, or at least acting, motivated the more that you build on these areas.
In my opinion, habit is the final key to unlock sustained motivation - as when motivation comes and goes you fall back on strong habits.