Here are different strategies you can apply in such situations:
The first one is to relax and get some rest from your practice.
With any form of practice or discipline, it is always good to give yourself a break, for instance in the week ends or by taking a week off from your practice every month.
This will give rhythm to your practice and build a healthy alternation between periods of activity and rest.
The goal is to relax in your practice and work with the forces of nature rather than against them.
The next strategy is to keep your practices exciting and refreshing:
Motivation has a lot to do with excitement.
If you want to establish some spiritual development techniques in your life, you need to change routines, sometimes improvise and have room for spontaneous direction shifts.
If you stick to just one practice day after day, you might maintain it for a few weeks or a few months but after a while, your mind naturally calls for renewal.
Listen to signs of boredom or tiredness and shift directions or rhythm when you notice them.
Be open to shift rhythm, add new practices, change routines or shift directions any time.
The key is to stay creative.
Stay awake and alert at all times.
Simply repeating exercises day after day because it is what is said in the book will quickly take you to a limit.
After a while, you need to raise your level of autonomy and "ownership" over these techniques and be able to play with them and in a way reinvent them in your own unique way.
Technique is not enough: you need passion and excitement to succeed with anything you try.