This is precisely chrstianity's constant mistake of two millenia. The "new testament" fulfills the "old" because it (the "new") says so. Ie, it assumes from the outset the authority of the "new testament" to authoritatively interpret the "old." This is the classic logical fallacy of "assumption of the consequent."
Joseph Smith claimed that he personally was prophesied in the chrstian bible. Most chrstians ridicule the idea. But his logic is no different from that of chrstians who "prove" the "new testament" by simply quoting its claims about J*sus' alleged fulfillment of Biblical prophecy. "I was prophesied in the bible, and we know this is true because I say so, and I was prophesied in the bible!"
The argument is no different than saying one plus one equals five, because I already believe that it equals five.
See the material here.
I gather it is all an issue of written authority to you. I can’t answer for others, but I look to the Jews who wrote the New Testament for my authority, Peter, James, John, Paul, etc. If Christ trusted them with his word, so do I. Case closed.
These were Jews who knew more about Old and New Testament issues than yourself, they lived 2000 years closer to the truth than you. Aren’t you a Noahide? what is your Jewish authority? Rambam? If so, my Jewish authorities preceded yours by about a thousand years or so.