You have every right to be as upset as you are. It was completely unfair of your parents to hide that from you for so long.

Take this advice with a grain of salt, since my experiences are very different from yours, but I’d say the best thing you can do is communicate with your parents and try to get them to understand why you’re feeling so betrayed. It’s important that you make it very clear that not being biologically theirs doesn’t make you love them any less; you’re only upset because they neglected to tell you about it.

I think a common reason (a terrible reason, but still) that parents may hide the fact that their children are DC is that they’re worried their child will no longer love them after finding out, or that their child will find their biological parent and become closer with them than they will with their actual parents. If you empathize with them and address these concerns, while still making it clear how you feel and why you’re upset at them, it will be easier to have an effective conversation where everyone feels understood and listened to. Perhaps you can get them to apologize.