Perhaps it would have been better if your parents had told you earlier about your adoption but in any case, this knowledge shouldn't make you feel as if something is wrong with you, or that you don't belong.
In fact, the opposite is the case. Listen: Your adoptive parents didn't have to choose you but they did! To them, you were so special that they wanted you to become part of their family for the rest of their lives. Instead of feeling like you don't belong, you should feel honored and have a great sense of belonging because they chose you.
After all, where would you be if they hadn't adopted you? We can't say, of course, but your situation might have been much different. But your parents have surrounded you with love (as you admit elsewhere in your letter), and instead of feeling shattered over this news, I hope you'll be thankful to them, and thankful to God. The Bible says, "Give thanks to the Lord, for he is good; his love endures forever" (1 Chronicles 16:34).
But I want to tell you about an even greater truth: When we give our lives to Jesus, God adopts us into His family. Think of it: He makes us His children! If you have never done so, I urge you to commit your life to Christ, for when you do, He will make you part of His family forever.