See how you emphasize what you want to emphasize?
Romans 3:19-31
19 Now we know that whatever the Law says, it speaks to those who are under the Law, that every mouth may be closed, and all the world may become accountable to God; 20 because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin. 21 But now apart from the Law the righteousness of God has been manifested, being witnessed by the Law and the Prophets, 22 even the righteousness of God through faith in Jesus Christ for all those who believe; for there is no distinction; 23 for all have sinned and fall short of the glory of God,
You have already agreed that Christians are not under the law and Paul is stating that there is not distinction between those under the Law and those who are not under tha Law since all have sinned and fall showt of the glory of God.
As to the Christians being under the Sabbath lets try this one more time:
Romans 14:1-8
14:1 Now accept the one who is weak in faith, but not for the purpose of passing judgment on his opinions. 2 One man has faith that he may eat all things, but he who is weak eats vegetables only. 3 Let not him who eats regard with contempt him who does not eat, and let not him who does not eat judge him who eats, for God has accepted him. 4 Who are you to judge the servant of another? To his own master he stands or falls; and stand he will, for the Lord is able to make him stand. 5 One man regards one day above another, another regards every day alike. Let each man be fully convinced in his own mind. 6 He who observes the day, observes it for the Lord, and he who eats, does so for the Lord, for he gives thanks to God; and he who eats not, for the Lord he does not eat, and gives thanks to God. 7 For not one of us lives for himself, and not one dies for himself; 8 for if we live, we live for the Lord, or if we die, we die for the Lord; therefore whether we live or die, we are the Lord's. NAS
Colossians 2:16-17
16 Therefore let no one act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day-- 17 things which are a mere shadow of what is to come; but the substance belongs to Christ. NAS
One who is weak in faith keeps the Sabbath and only eats vegetables, while one who is strong in the faith is free to eat what he wills and observes every day as unto the Lord.
If is unfortunate you would try to place yourself under the yoke of slavery that is the Law.