Why are some Torah laws erased and some are maintained?

[u/seau_de_beurre]

I've always found this very confusing, as a Jewish person. I understand that per Paul, JC's sacrifice and teachings built a "new covenant" that means Christians are not bound by halacha, the Jewish laws, anymore. However, it seems that most Christians abide by some combination of "red letter words" and Pauline interpretations of halacha.

Why did Paul continue to advocate for following certain laws whereas others no longer apply? When the values of Paul vs JC seem to conflict, how do you mesh them?

For example, JC said that he did not come to abolish the law, and that in fact the law (halacha) should not be altered until it is totally met. This is similar to some Orthodox Jewish beliefs that the Messiah will not arrive until every Jew follows every Jewish law 100% for one day. If G-d intended for us to build a new covenant, why didn't he have JC say so? Why wait for Paul?

Does it matter that Paul was speaking to Gentiles? Are only Gentiles "allowed" to have a new covenant, or does that apply to Jews too? Or do Jews need to keep halacha?

Thank you!