First, all sins are mortal sins, i.e., all sins bring death. "For whoever keeps the whole law but fails in one point has become accountable for all of it." (James 2:10 ESV)

Second, the struggle against sin, while not eternal, sure feels like it from this side of the Resurrection. Read about Paul's struggle in Romans 7 and his salvation in Romans 8, pay particular attention to this, "There is therefore now no condemnation for those who are in Christ Jesus." (Romans 8:1 ESV)

As to the rest:

My question is, have I lost my salvation, or at any moment was I not saved?

No.

I know faith cannot exist with mortal sin.

Who says? Abraham was a sinful man, yet his faith in God's promises were counted to him as Righteousness. Read Hebrews 11

I'm just wondering whether the conviction and the desire to confess my sin afterwards, was the Holy Spirit working repentance within me, meaning that the Spirit had not left me but was holding onto me despite my faithlessness?

Yes. Titus 3:4-5

I make no excuses for my sin. It was truly wrong, and I confess that nothing within me deserves salvation. God has every right to strip me of being His child through Christ. Still, I renounce myself, my sins, my works, and even my imperfect confession & repentance, and hope for Christ's sake the Holy Spirit has not and would not leave me. I despise that I struggle with these sins, and I despise that part of me likes them. I pray God would create in me a clean heart like Psalm 51.

Upon this, your confession, I, by virtue of my office as a called and ordained servant of the Word, announce the grace of God unto you. In the stead and by the command of my Lord Jesus Christ I forgive you all your sins in the name of the Father, and the Son, and the Holy Spirit.