you can't.
only God can change a heart -- in the psalms, David doesn't say "i will make my heart pure," instead he says "Create in me a pure heart" ( Psalm 51:10 )
and in Ezekiel 36:26 -- the Lord says:
"I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh."
it's not Israel that God demands change their heart, but he promises that he will change them.
let the spirit live in your heart, and it will teach you the things of God. a heart that wants to please God is a pure heart -- already he is working in you! have faith, and patience, and understand that "the heart is deceitful above all things and desperately wicked; who can know it?" (Jeremiah 17:9) -- don't be discouraged, what you describe is the same experience we all have, a spirit that is willing and a flesh that is weak.
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