From time to time genuine believers may have disturbing doubts about their salvation. Such doubts are most often because we focus upon ourselves and not on Christ. Trusting in ourselves leaves room for all manner of doubt. Our assurance of salvation is in Christ alone. It's knowing who saved you.
Faith in Jesus Christ is our assurance, testifying, "I know whom I have believed and am persuaded that he is able to keep what I have committed to him until that Day" (2 Timothy 1:12). It's faith in Christ alone. He alone has the authority and power sure to save and keep us.
Do not focus on your work for Christ, but his work for you and in you. The grace of Christ initiates your salvation. What he begins, Christ is more than able to finish. "Being confident of this very thing, that he who has begun a good work in you will complete it unto the day of Jesus Christ" (Philippians 1:6). Your salvation is sure by faith in Jesus Christ our Lord. What he began in you continues even to the final day. That is the day of Christ, when he returns.
Believe on the Lord Jesus Christ to be saved and sure (Acts 16:31). You are saved by what Christ has done for you at the cross and through his resurrection (1 Corinthians 15:1-4). You are saved by what Christ is doing in you and through you (Colossians 1:27; Hebrews 13:20-21). Christ alone saved you and not yourself. Assurance of salvation is by faith in Christ alone.
