This is totally natural. Not every part of your body develops at exactly the same rate. So, one breast can start developing earlier than the other. It is actually pretty common. So, you are not stupid, and you should not feel self-conscious about it. Plus, it is hardly noticeable in most cases.
It's very normal. You are not stupid. It's possible to get custom bras with different size cups too.
Hey my left one is smaller than my right!
No, you are certainly not stupid and this situation is not all that rare, either.It sounds as if your right breast is now no longer going to catch up with the size of your left breast by responding to circulatory hormones at this relatively late stage.