To cultivate the will to do something (anything, no matter how trivial), you must understand why that thing you want to do is important or urgent to do. In other words, you must understand how you view the world (i.e. you must understand "your" ideas, expectations, and beliefs about the world, its parts, the relation between its parts and your role in such framework) in order to comprehend what your values are. Your values are born from your world-view, and indicate the things/facts/entities/events/ideas that you consider worthwhile.
Once you undestand deeply this theory of how emotions work, then you can start to see how "our" will operates: we do things when we are convinced that effort is worth being invested in the things done.
Now, the next step is to "convince" your emotions and values about the importance of letting go. To do so, you have to integrate new information about the Dhamma, and why letting go is such and important part of the path. This information can be reached by your own constant reflection and thoughts; by studying the suttas; or by listening or reading from other students/disciples of the Buddhadhamma.
To being able to let go, first you have to learn, at least theoretically, that feelings, perceptions, and thoughts are not "yours", neither "you". They are, instead, just habitual mental patterns, born from ignorance (i.e., from ignoring the Dhamma). Once you stop identifying with feelings, perception and thoughts, you will be able to start seeing them just by what they are: inertial habits of thought. Then, letting go becomes the natural progression, almost a by-product of all previous reflection and learning.
This "by-product" is the result of various factors interacting with each other. How to cultivate such factors? The Noble Eightfold Path help us to train each one of them. The key idea to keep in mind is that the training is gradual and progressive. The release of the mind takes its time, and goes according to the effort put into the practice of the whole Eightfold Path. As your emotions get reasons and evidence of the truth behind the Buddhadhamma, then your deeds will gradually go towards where this new emotional evidence/inormation leads you: liberation.
In sum, to exert effort is to have the will to take some course of action. But to have that will, you must have conviction of the relevance of such course of action in which you'll put your effort. That conviction grows from progressive understanding of the Dhamma.
Because of all the above, it is said that Right View is the forerunnner of the practice. See in this link for more details.
Have a wonderful day.