So, if I doubt in my heart, does that mean that God will not work in my life.
Not at all.
Our Covenant with God is not based on what we do, but what Jesus has done.
Yet in even the Old Testament, under the Old Covenant, God's saints doubted from time to time, yet that never prevented God from blessing them.
Consider Jacob, who had lost his son Joseph, then Simeon, and feared also losing Benjamin:L
"And Jacob their father said unto them, Me have ye bereaved of my children: Joseph is not, and Simeon is not, and ye will take Benjamin away: all these things are against me." (Genesis 42: 36)
Yet later, Jacob would see his son Joseph, at that point the ruler of Egypt:
"30And Israel said unto Joseph, Now let me die, since I have seen thy face, because thou art yet alive." (Genesis 46: 30)
And even when Jacob said this, he still lived another seventeen years (Genesis 47: 28).
Beloved King David also despaired in his heart, and still he became King, as God had anointed him to be:
"1And David said in his heart, I shall now perish one day by the hand of Saul: there is nothing better for me than that I should speedily escape into the land of the Philistines; and Saul shall despair of me, to seek me any more in any coast of Israel: so shall I escape out of his hand." (1 Samuel 27: 1)
Despite the thoughts of David's heart (that he doubted he would live), David did ascend the throne and become king (2 Samuel 5: 12)
Yet can we get a heart which never doubts? Yes, through His New Covenant
God gives us a new heart:
"10For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
11And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
12For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more." (Hebrews 8: 10-12)