Istighfar is also known as Astagfrullah, is the act of seeking forgiveness from Allah. We ask forgiveness from Almighty Allah by repeating the Arabic words (Allstaghifirullah), which means "I seek forgiveness from Almighty Allah. " It means to get strength from Allah to refrain from sin and evil to be able to display this strength to refrain from sin. We should always seek forgiveness from Almighty Allah as He is the Most Merciful

People often forget the importance of the simple yet powerful Dua Istighfar. .i.e .saying "Astagfrullah " It is the Sunnah of our Prophet ( PBUH ) to seek Istighfar. Istighfar is seen as portraying the strength and support of Allah which leads to attaining closeness to Allah Almighty and redemption. (Q51: 16-19)

And Q2:222, Q11;3 .The Prophet muhammad ( PBUH ) said : "Whoever is always in the state of affairs, then Allah will give the joy of sadness, and the spaciousness of its narrowness and give it the rizq from an unexpected direction"(Ibn Majah)
Q,71:10-12
Hence, Istighfar gives strength to the prayer; Allah Almighty promises those who do Istighfar regularly to give a good life continuously throughout the life of the world until if comes to an end and gives various advantages and Benefits to those who always Istighfar and begs from Him .We will get more spirit and strength by saying Istighfar everyday

Also see (The story about Istighfar in connection with the life of (Imam Ahmad Bin Hanbal) Rohimohullah . Sheik ul-Islam or Imam of Ahl-Sunnah. "Al-Jumuah magazine, vol 19, issue 7""