This is the first of a short series of reason someone might doubt their salvation followed by sources and statements that have helped me.
The most obvious is that you aren’t saved. If this is the case, then you need to repent of your sin and turn to Christ as the only source of salvation. Turning to Christ means you believe on His life, death, burial, and resurrection as the source of your salvation. You realize that your sin is deserving of eternal damnation in hell and there is absolutely nothing you can do to pay the penalty. You know that you are desperately lost and that your only hope is the faith that God is giving at the present moment to depend upon the substitutionary atonement for your sin. The whole picture is salvation by grace alone. You can know that He is calling you if you have the faith to believe. You can know that He has given you the faith to believe if He is calling you.
II Corinthians 13:5
5 Test yourselves to see if you are in the faith; examine yourselves! Or do you not recognize this about yourselves, that Jesus Christ is in you—unless indeed you fail the test?
And you were dead in your trespasses and sins, 2 in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. 3 Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest. 4 But God, being rich in mercy, because of His great love with which He loved us, 5 even when we were dead in our transgressions, made us alive together with Christ ( by grace you have been saved), 6 and raised us up with Him, and seated us with Him in the heavenly places in Christ Jesus, 7 so that in the ages to come He might show the surpassing riches of His grace in kindness toward us in Christ Jesus. 8 For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; 9 not as a result of works, so that no one may boast. 10 For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand so that we would walk in them.