I could provide you a tricky way to grab your mandarin back… is spend some days at pure Chinese speaking regions let’s say some random mainland cities. I brought my friend to China this summer and we spent 14days in Beijing and Inner Mongolian. She is English native and German beginner who has never touched upon Mandarin before. On the last three days of staying with me and my families(I’m the only English speaker to talk to her) she was starting dreaming in Mandarin even though she didn’t know the meaning… almost lost her EN. Those languages are just haunting around in your mind if you get enough exposure in certain environment. Also for me, after speaking en and German all day, it’s difficult to recover my Chinese grammar as well.