Why do you say in your pastoral prayer that we "constantly sin against God"? Do not Christians have victory over sin? And if sin is a lack of conformity unto or transgression of the law of God, then how can it be said that we are sinning all the time? I mean, the Ten Commandments are very specific, and I can’t see how I can be breaking them all the time.

First of all, It is true that Christians have victory over sin. Christ has freed us from the bondage of Satan and the dominion of sin (cf. Col 1:13; Acts 26:18; Rom 6:14). While an unbeliever is unable not to sin, a believer is able not to sin. An unbeliever is inclined to sin, but not a believer. Sin has no longer dominion in a believer. It is checked, controlled and mortified by the expulsive power of the new principle of grace. However, the prophet Isaiah in his prayer unto God said: "But we are all as an unclean thing, and all our righteousnesses are as filthy rags" (Isa 64:6a). Notice that it is not "our sin" but "our righteousness" that is likened to filthy (menstruous) rags. In other words, Isaiah, under divine inspiration, is saying that even the best deeds are polluted or tainted with sin. Who does ‘we’ refer to in this statement? It refers to the Jews, the Christians of the Old Covenant, and it includes Isaiah himself. There is no question therefore, that the principle of Isaiah 64:6 is applicable to all man: believers and unbelievers. Even the best works of believers "are imperfect and defiled in the sight of God" (WLC 78). This is what I mean by "we sin constantly." When we understand this, we understand one of the reasons why we say our salvation (including sanctification) is wholly and totally on account of Christ and that "We cannot, by our best works, merit pardon of sin, or eternal life, at the hand of God" (WCF 16.5). This is not to say that good works are therefore of no use for Christians. Our Confession puts it well:

WCF 16.6 Yet notwithstanding, the persons of believers being accepted through Christ, their good works also are accepted in Him; not as though they were in this life wholly unblameable and unreproveable in God’s sight; but that He, looking upon them in His Son, is pleased to accept and reward that which is sincere, although accompanied with many weaknesses and imperfections.

Secondly, sin is indeed relative to the Moral Law of God, and, as our Catechism teaches, is either a lack of conformity unto or transgression of the Law (WSC 14). However, remember that the Ten Commandments is a summary of the Moral Law and not the Moral Law itself, which is very broad. We must not think that we are sinning only when we can pinpoint that we have broken a particular commandment. I am sure we do break one or more commandments at any time, but the hardness of our heart and the corruption of nature will always cloud our judgement. But consider the most general summary of the Moral Law as given by our Lord: "Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment" (Mk 12:30). Can you say that you do constantly and perfectly love the Lord your God with all the faculties of your being? I know I cannot. Remember that original sin is sin; and by the wisdom of God, the corruption of nature, from which an inclination to sin arises, does remain in those who are regenerate (cf. WCF 6.4–6.5). When we understand this, then we would place no confidence in the flesh. We will never forget to watch and pray lest we fall into temptation, but we rejoice in Christ Jesus saying, "Thanks be to God, which giveth us the victory through our Lord Jesus Christ" (1 Cor 15:57).