Liberation is the end of a very long road, it is not achieved overnight. A true yogi practices and meditates all his life to attain it.
At the beginning it is very difficult to meditate for long periods: the body starts to itch, the mind starts to wander.
That is why it is very important to know oneself and to be humble about one's limits... Practice and everything will come... Sri K. Pathabi Jois used to say. Start with 5 minutes a day for yourself and your meditation and gradually those 5 minutes will turn into hours.
Remember your breath. It is the key to attaining liberation and staying in meditation for long periods. The rhythm of your breath changes depending on what you are doing. Try to notice its flow during meditation and try to reproduce it when you have trouble concentrating on your practice.
The benefits of Yoga
Lecture on Karma - Mani Raman
About the importance of breathing
The personality of a Yogi
The length of a class
How to achieve the Jivamukti state
Focus your mind during the practice
Is Ashtanga Yoga for everyone?
The path of Yoga