If you are feeling unmotivated to practice yoga, there are some things you can do to kick your motivation back into gear. One thing you can do Is shorten your practices. Instead of doing a full practice, try cutting down to only 10 or 20 minutes, 30 minutes max. This will be plenty of time to help your body feel the effect of a full practice without doing a full session. Even if you just do a short, seated session you will feel more relaxed.
Find a regular and consistent time to dedicate to your practice. Put this time in your calendar so you always know that it is time to practice. Do not schedule anything else during this time and keep your distractions to a minimum. Focus all of your energy on relaxing.
You can also try practicing yoga somewhere else than on your mat. This will help you gain a wider sense of your practice and what you are hoping to gain from it. Yoga teaches you how to breathe properly and how to calm yourself when you are starting to get worked up or angry. Use these skills when you feel yourself start to get angry or frustrated, you will realize how beneficial yoga has been for you.