The only way to fight doubts is the word of God. Jesus even used Scripture to fight Satan himself, and He wrote it!
( John 1) Eve added to the word of God in the Garden, so she did not take the time to commit to memory what God had said. (compare 2:15-17, 3:1-5) Satan saw this and used deception to attack her as a weaker link to get to Adam, IMO.
Satan will always bring doubt upon the word of God, that's how he and his tools (the unsaved and professing believers) work. They always work the same. Know the word, know why you can trust it, and use it when people and doubts attack you.
Love for God casts out fear of punishment. The more you study about God and know His character, and His plan of redemption, the more you trust and love Him. After all, He chose you!
This can only happen by general Bible reading, study of doctrines and subjects, and memorizing key Scriptures that will help you fight the one that causes you to doubt. Many false teachers out there and on the forums, so beware.
We are like a baby and need daily feeding of the word to grow properly and we have to be holy, to separate ourselves from things that might drag us down. This be people, places, and things.
To the extent you do all these things will be how quickly you grow and mature in the faith. No easy paths on the pilgrims progress.
Hope this helps!
Hold fast, stay the course!
1Therefore, rid yourselves of all malice and all deceit, hypocrisy, envy, and slander of every kind.
2Like newborn babies, crave pure spiritual milk, so that by it you may grow up in your salvation,
3now that you have tasted that the Lord is good.
13Therefore, with minds that are alert and fully sober, set your hope on the grace to be brought to you when Jesus Christ is revealed at his coming.
14As obedient children, do not conform to the evil desires you had when you lived in ignorance.
15But just as he who called you is holy, so be holy in all you do;
16for it is written: “Be holy, because I am holy.”
a
17Since you call on a Father who judges each person’s work impartially, live out your time as foreigners here in reverent fear.
18For you know that it was not with perishable things such as silver or gold that you were redeemed from the empty way of life handed down to you from your ancestors,
19but with the precious blood of Christ, a lamb without blemish or defect.
20He was chosen before the creation of the world, but was revealed in these last times for your sake.
21Through him you believe in God, who raised him from the dead and glorified him, and so your faith and hope are in God.
22Now that you have purified yourselves by obeying the truth so that you have sincere love for each other, love one another deeply, from the heart.
b 23For you have been born again, not of perishable seed, but of imperishable, through the living and enduring word of God.
God is love. Whoever lives in love lives in God, and God in them.
17This is how love is made complete among us so that we will have confidence on the day of judgment: In this world we are like Jesus.
18There is no fear in love. But perfect love drives out fear, because fear has to do with punishment. The one who fears is not made perfect in love.