As strange as it may sound, rewards for performance aren't your primary motivation to get started and get through your work. First, you have to feel good.
Motivation is About Feeling
The quick answer to motivation (or inspiration) is that you have to start with feeling good in some way in order to perform. At all. And you have to be able to continue feeling good enough to complete the task. You also have to have a high enough level of good feeling in order to perform well.
You don't have to be elated. Or feel good about whatever action it is you have to perform (although that helps.) You just have to feel "good enough," have the energy, both physically and emotionally, to get started and stick it through.
Regardless of the task you have to do, you'll resist or perform badly if you are tired, hungry, wet, cold, angry, anxious, depressed or otherwise just not feeling good.
What You Need to Feel Good About
In general, you simply have to feel good about something significant and be able to attach those feelings to any specific action.
People who are happy perform better at almost anything than people who aren't. So, if you are happy in general with your employer, you'll probably perform well on your various tasks, including tasks you don't like. If you hate your boss, organization or team, your performance will falter although it may be on tasks that you otherwise like.
So, you have to feel good about something that's meaningful enough to extend good feelings into whatever your circumstances and tasks.
You have to feel good about yourself. Or the organization you work for. Or the people you work with. Or the work that you do. Or the beneficiaries of your work. Or...whatever it takes to ring your bell and get you going.
How does that look in action?
Think for example about "Doctors Without Borders." They are able to deliver very needed medical services, in dangerous and uncomfortable conditions, to citizens of countries where they may hate the governments. They can do it because they are motivated by love of their patients and/or joy in being able to give such value to society. Many say they've done some of their best work in that volunteer position. They find lots to feel good about.
How To Feel Good Enough
As I suggested earlier, you need to make sure you've taken care of your basic needs. Be rested enough, eat regular meals, be organized enough, have people you care about and who care about you. Find work in an acceptable atmosphere, in an acceptable line of work (by your standards.) Be able to make choices about things that are important to you. Develop your self-esteem.
These are among the requirements most people have as a basis for performing well enough to be paid for their work. And as a basis for learning and growth. There may be different or additional requirements for you.
Everything you add beyond that is what sets the motivation for higher levels of performance. Get a great social life. Get training and education. Develop great skills that you're proud of. The better you feel, the more motivation you'll have to do anything. Perhaps things you once thought impossible.
A Note For Managers
You've probably discovered that you can't always get people to do what you want them to do, even when it's in their best interest. Even when you give them money and/or other rewards that far exceed the worth of whatever you want them to do.
Too many managers believe that the rewards you'll get for your performance should be enough. It's true that anticipating a reward is part of motivation. But not feeling "good enough" and trying to perform anyway is a punishment experience. That stifles performance.
If you want better performing employees, make sure they get what they need to feel good enough. Create the best working atmosphere you can. Give your employees the tools and support they need to get the job done. Then see what you can do to help them actually find happiness in working for you.