This sounds like the classic case of the temporary boosting of QDF. Now that temporary boost has worn off you must do solid promotional work to truly earn the higher rankings you seek.
If you search this forum you will find hundreds of threads describing your exact situation and the feeling of being in the imaginary sandbox once the temporary boost of QDF wears off.
By the way, there is no real sandbox, it is just an imagined place that your site goes when you don’t understand what is going on. It’s quite common for new webmasters to feel like their website has been placed in a sandbox when they don’t understand what is going on. It’s all in your mind, you simply haven’t beat your competitors, you’ve more work to do.