Verses 12 & 13 is about obeying Paul in having that mind of Christ in worship in honoring the Son to glorify the Son in order to honor and glorify God the Father. The working out our salvation with fear & trembling is to recognize we are His workmanship as we are to trust Him to do His work in us and not resort to our own power by the deeds of the law for which we would be denying Him as bale to perform by faith in Jesus Christ alone to finish.
Philippians 2:5 Let this mind be in you, which was also in Christ Jesus: 6 Who, being in the form of God, thought it not robbery to be equal with God: 7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: 8 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. 9 Wherefore God also hath highly exalted him, and given him a name which is above every name: 10 That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; 11 And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father. 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.
Philippians 1:6 Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ: 7 Even as it is meet for me to think this of you all, because I have you in my heart; inasmuch as both in my bonds, and in the defence and confirmation of the gospel, ye all are partakers of my grace. 8 For God is my record, how greatly I long after you all in the bowels of Jesus Christ. 9 And this I pray, that your love may abound yet more and more in knowledge and in all judgment; 10 That ye may approve things that are excellent; that ye may be sincere and without offence till the day of Christ. 11 Being filled with the fruits of righteousness, which are by Jesus Christ, unto the glory and praise of God.
That means we are to follow Him by faith in Jesus Christ as our Good Shepherd to help us to follow Him; not by keeping a commitment nor a promise. Those who do, are to repent by asking Jesus to forgive them and to set them free from their yoke of bondage, so they can rest in Him and all His promises to us under the New Covenant.