Well you've made it to age 19 and avoided getting dunked, that's a good thing. The best way to handle having a worldly boyfriend is to keep it on the down-low as much as possible. Hopefully he will realize you are not embarrassed by him and that its not because you dont want to be seen with him, its just that your family is in a wacky religion. I am sure there are dozens of people here who can easily explain it to him if he has a hard time understanding your situation.
Elders do not have Super powers, the only way for them to find out you are dating or whatever is if you tell them. Well, or if someone else tells them. Loose lips sink ships. Even then, you are 19, you are not baptized, they have zero control over you.
Let me repeat this: They have zero control over you.
Unfortunately, they don't always realize this fact. But that's their problem, not yours.
It sounds like your main concern is how your parents will react. I don't know your whole situation, but once you become financially independent and get out from under their roof you will have much more freedom and your fade can progress much more rapidly. I'm not saying you have to move out onto the street or with a boyfriend who may or may not be the right guy just this instant, but that should be your longterm goal. If you have to be a little secretive about your life and your personal affairs until that happens, well, call it theocratic warfare.
Did I mention that as an adult who has never been baptized, the elders have zero control over you?
No Apologies