You can have doubts but how do you resolve them?
By being reassured of the promise of free everlasting life by believing Jesus is the son of God.
Thats how you and every believer restores them selves from doubts.
Free everlasting life means free everlasting life , not conditional part life but free not of works of law
or self works Life, but free unconditional freely given by God's free gift of Grace by the gift of believing Jesus is the son of God everlasting life.
Full of grace and truth.