Q: Aren't you going back under the Law?
A: The phrase "under the Law" essentially means that a person is condemned by the Law because they have transgressed the Law.
If an individual is saved by grace through faith in Messiah Yeshua (Ephesians 2:8) then they no longer stand condemned by the Law (Romans 8:1). Obeying the Law as a consequence of being saved is what we were created for (Ephesians 2:10).
To assign a meaning other than what Scripture has provided to the phrase "under the Law" is to perpetuate error. Psalm11918.org has provided an article regarding What Scripture Says About Being Under the Law to examine every instance of the phrase "under the Law" used in Scripture and derive a proper understanding of that phrase from Scripture itself.
Nobody is justified by the works of the Law (Romans 3:28, Galatians 2:16)... ever. If a person is attempting to earn, merit, or gain salvation by works of the Law then they are not living by faith and remain under the condemnation of the Law.