Consider that His mercy endures due to His priesthood and sacrifice. He intercedes on your behalf, being your Advocate with the Father. One sacrifice for all sin. Not like the priests of old, doing daily sacrifices for daily sin, but once did He sacrifice Himself (as the Lamb slain from the foundation of the world). For if His blood were like the blood of bulls and goats, Christ would've suffered since the foundation of the world (being the sacrifice for sin).
What I am trying to express here and may God give you understanding (because the Lord leads us into all truth) is that you are forgiven. Your sins have experienced remission through the shed blood of Christ. The mercy you speak of is not as if the Lord is in consideration of whether to forgive you or not. You are forgiven in Christ Jesus. This is a covenant reality, a covenant where He says He has written His laws on our hearts and remembers our sins no more.
These truths are found in the book of Hebrews.
You say that Jesus said to work out your salvation with fear and trembling, but I wish to remind you that
Paul said this.
Philippians 2:12-13 King James Version (KJV)
12 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
13
For it is God which worketh in you both to will and to do of his good pleasure.
What does it mean to "work out your own salvation
with fear and trembling"? And why does verse 13 say "for it is God which worketh in you both to will and to do of his good pleasure" in reference to verse 12? It is God doing the work in you to do of His good pleasure, so what does fear and trembling have to do with this? Well, let me show you another scripture using "fear and trembling" so that you may discern its meaning.
1 Corinthians 2:1-5 King James Version (KJV)
2 And I, brethren, when I came to you, came not with excellency of speech or of wisdom, declaring unto you the testimony of God.
2 For I determined not to know any thing among you, save Jesus Christ, and him crucified.
3 And I was with you in weakness, and
in fear, and in much trembling.
4 And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power:
5 That your faith should not stand in the wisdom of men, but in the power of God.
Notice that Paul was putting emphasis not on his own ability (such as speech, man's wisdom, enticing words, etc) but on the demonstration of the Spirit and of power. The fear and trembling was a distrust in his own abilities and a reliance on the Lord. Now, let me show you the Greek (if thats okay?).
https://biblehub.com/greek/5156.htm
Thayer's Greek Lexicon
STRONGS NT 5156: τρόμος
τρόμος, τριχοῦ, ὁ (τρέμω), from Homer down, a trembling, quaking with fear: Mark 16:8; μετάφοβοῦ καί τρόμου, with fear and trembling,
used to describe the anxiety of one who distrusts his ability completely to meet all requirements, but religiously does his utmost to fulfil his duty, 2 Corinthians 7:15; Ephesians 6:5; Philippians 2:12; ἐν φόβῳ καί ἐν τρόμῳ (Isaiah 19:16), 1 Corinthians 2:3 (φόβος and τρόμος are joined in Genesis 9:2; Exodus 15:16; Deut. (); , etc.; ἐνφόβῳ ... ἐν τρόμῳ, Psalm 2:11). (Synonym: cf. φοβέω, at the end.)
Another scriptural example...
Ephesians 6:5 King James Version (KJV)
5 Servants, be obedient to them that are your masters according to the flesh,
with fear and trembling, in singleness of your heart, as unto Christ;
The emphasis is that while we do our best, it is God who works in us. This isn't a verse (Philippians 2:12-13) filled with condemnation (or a fear tactic), rather it is teaching us to rely on God who works in us. Who is the Author and Finisher of our faith? It is the Lord. We understand that we fall short of God's glory, and for this reason we are dependent on Christ.
This verse isn't mean to incite fear, but confidence in God. Praise God that He works out in us to do His will and the things that please Him. He has equipped us and leads us as we are saved unto good works.
I hope this has helped, in some manner.