A great source of our inner strength comes from our spiritual and religious beliefs. Jesus and Buddha both drew their strength from a Higher Power, knowing that they were never truly alone during their trials. This has been my own personal source of strength throughout the years as well. It was my spiritual beliefs that delivered me from the deepest ruts in my life, and continue to give me a shoulder to lean on. If your beliefs hold no true comfort for you, and are not a source of strength for you in your daily life, then perhaps you should explore other beliefs or paths until you find one that eases your soul.
Music can be a great source of inner strength as well.
When you are facing a difficult situation, perhaps the lyrics of one of your favorite songs will pop in your mind and ease your worries. Or perhaps singing or playing an instrument gives you the power to face obstacles and builds upon your inner power. Physical exercise can also be a great link for this internal asset. Because the physical and mental are tied together, increasing the strength of one often helps increase the strength of another. Exercises such as Tai Chi and Yoga are specifically geared to build both inner and outer strength.