This a difficult one to tackle and understand.
I am in the midst of reading a book by Charles Spurgeon: The Power of Prayer in a Believer's Life. He presents the struggle in this way.
And sweetest thought of all, every covenant promise has been endorsed and sealed with blood , and far be it from the everlasting God to pour scorn upon the blood of His dear Son. The covenant is ratified with blood, the blood of His own dear Son. It is not possible that we can plead in vain with God when we plead the blood-sealed-covenant, ordered in all things and sure. Heaven and earth shall pass away, but the power of the blood of Jesus with God can never fail. It speaks when we are silent and it prevails when we are defeated. Better things than that of Abel does it ask for (Heb. 12:24), and its cry is heard. Let us come boldly, for we bear the promise in our hearts.
I pray that this encourages you. I strongly recommend this book to anyone desiring to better their prayer life; the whys and hows.
Praise Jesus for our compassionate Creator!