I have an adopted bipolar child as well, only today she is a young adult. She wasn't particularly prone to lying when she was younger, although she did have a lot of problems with impulsivity, hyperactivity and moodiness...among other issues. I would also describe her as "sweet." She really has a good heart. AND, I also struggle with the idea that if she knows the difference between right and wrong, why does she chose wrong so frequently? In our case, I think it is largely an issue of impulsivity. At times, I think she also had some problems with entitlement. But that impulsivity really trumped it all.
And the difficult thing with her being adopted, trying "tough love," on these kids sometimes confuses them. They think they are unwanted and fear abandonment.

Very generally speaking, medication and therapy was of help to her. This took a lot of effort and ongoing care.

Consequences for misbehavior was a part of her learning process. Speaking again very generally....we did make sure there was some consequence for poor decisions. We often did not lower the boom (just an expression) on her as we might for another child who did not suffer from her affliction. We tried to make the connection clear. However, we avoided giving her a "pass." IMHO, the laws of cause and effect need to be demonstrated and upheld. (Believe me this is all draining, complicated, confusing and difficult).

Please make sure you take special care of yourself and your relationship with your spouse (or significant other...whatever the case might be). It is highly, big time doubtful that you messed up in any way. All you can do is get help from the experts and keep moving forward. Having a child with special needs is taxing on the mind, spirit, body and soul. My heart goes out to you.

p.s. That book, The Bipolar Child, was my lifeline when my daughter was little. It gives top notch advice on meds, school, life at home...it all. It's really a must!!!! If you don't have it already, I would get it as soon as possible.