My great-grandparents adopted me a long time ago and are very old. I’m 19 but forbidden to talk to my birth parents. My birth mom was a drug addict and alcoholic, and my birth dad was a Wiccan (awful in my Southern Baptist great-grandparents’ eyes). More in comments.
I agree with Lori Jones. Since you are 19 you are old enough to make your own decisions and not share everything with your great-grandparents. You can still honour them by coming home regularly and sharing the “acceptable” parts of your life. And by all means let them know how much you appreciate them taking you in when your birth parents were unable. Their advanced age most likely makes them unable at this point to understand your need to connect with your birth parents, or even to accept that your birth parents may have cleaned up their act. On top of that, they may feel afraid of losing your loyalty—jealous—after all the work of raising a child in their old age. That they did it was admirable and they probably heard it from every side at the time. Your reconnecting with your birth parents may feel like a slap in their face, though it shouldn’t. They should rejoice that their grandchild has changed. Since that is not the case, the best we can advise is not to inform them of your connections, and to be prepared to deal with it when and if it does come to their attention.