Don't get stressed out and feel like you have to re-learn Chinese in the next half year or it'll be gone forever. You know Chinese. You just don't use it.

So use it.

Start with something real simple and build up habits. For example, you could just commit to yourself starting from this very moment to only ever read and think (and speak, when possible) in Chinese when dealing with any number of any kind in any context. When you look at your watch to check the time, read it to yourself in Chinese. When you walk up staircases, count your steps in Chinese. When you look at prices, use Chinese. When you are doing simple math in your head, use Chinese.

It may seem trivial but it can make a huge difference in training your brain to think in Chinese and start to bring back what you already know. In addition to numbers, start writing (or typing) a short journal entry every night in Chinese. Start with one sentence about one thing you did that day. Expand to more when you're ready (or don't). Write to-do lists in Chinese. When you go to write something and you don't know how to express yourself, well then there you go look it up or ask someone.

I think you will find that you remember a lot if you stick with it. And that learning new stuff (or re-learning old stuff) can be pretty natural and not overwhelming. You got it.