On the contrary, "keeping Sunday" is not an essential issue. And because you are "pissed off" over it, spiritual immaturity is not your only problem - your emotional immaturity will guarantee that you will stay that way.
Some people are perpetually pissed off. God must be very pleased that they spend their life seeking out petty things to be pissed off about.
Churches do things all the time that I think are silly - rather than getting pissed off, I just don't go to those churches.
You are in error.
Your position is not orthodox or Reformed. You are a heretic.
From the Westminster Confession of Faith:
VII. As it is the law of nature, that, in general, a due proportion of time be set apart for the worship of God; so, in His Word, by a positive, moral, and perpetual commandment binding all men in all ages, He has particularly appointed one day in seven, for a Sabbath, to be kept holy unto him:[34] which, from the beginning of the world to the resurrection of Christ, was the last day of the week: and, from the resurrection of Christ, was changed into the first day of the week,[35] which, in Scripture, is called the Lord's Day,[36] and is to be continued to the end of the world, as the Christian Sabbath.[37]
VIII. This Sabbath is to be kept holy unto the Lord when men, after a due preparing of their hearts, and ordering of their common affairs beforehand, do not only observe an holy rest all the day from their own works, words, and thoughts about their wordly employments and recreations,[38] but also are taken up the whole time in the public and private exercises of His worship, and in the duties of necessity and mercy.[39]