Romans 12:2 (KJV)
And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what [is] that good, and acceptable, and perfect, will of God.
Proverbs 23:7a (KJV)
For as he thinketh in his heart, so [is] he...
The question is then, what is the heart? The heart is where the judgements of your experiences are located - the subconscious if you will - and is stored in every cell of your body. You're brain is a CPU doing calculations, processing the input, but it's the heart that matters the most. To change your heart you need to retrain your CPU to think on the things of God, but it's the deep things of your heart (not just the pumping organ). Many believe because they pray a prayer or heard the Name of Jesus they're saved, but their heart hasn't changed. To get there it does begin in the CPU of your head and change your head thinking by repenting - metanoeo (change mind) - so that it can change your heart.
So can we trust the heart?
Jeremiah 17:9 (KJV)
The heart [is] deceitful above all [things], and desperately wicked: who can know it?
The word deceitful is really the word for foot-tracked, as in "he walked all over my heart" and the words desperately wicked is actually the word for weak, frail, very sick. So what's this mean? Our hearts are broken!
This is why Jesus came:
Luke 4:18 (KJV)
The Spirit of the Lord [is] upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised,
We are broken (because of sin, and our life experiences) and therefore our life judgements (stored in our "heart") affect our CPU thinking so we are blind to the truth.
2 Corinthians 4:3 (KJV)
But if our gospel be hid, it is hid to them that are lost:
That is until we get saved... then we have the ability to change our old man (heart) because we are a new creation.
Proverbs 17:22 (KJV)
A merry heart doeth good [like] a medicine: but a broken spirit drieth the bones.
Just understand that positive thinking is NOT faith. Faith comes from the heart, not the head.