I agree with @valentineborgne. You should make a workout that will work the abs, back, calves, and legs (so pretty much a full body workout lol). To remember things you can write them down so you know what you were just working on and can think about how to improve that skill. I would also look at some books or articles about horses and riding and just do a review. You may come back knowing a few new facts too! Overall though you should be able to get back into riding pretty quickly if you are exercising regularly. It’s mostly muscle memory, how you sit, where your legs go, etc. and you have been around horses for a while so your mind knows how to react to things. Hope this helps and hopefully you get back to riding soon!
To work out without riding you can make one or two full body workouts a week, keep your back right while seated and make your abs contract. You can also cycle without being seated, it will spread your tendons.
Don't worry about forgetting, it is an automatic reflex that will come back really soon, just like riding a bike, you never forget how to. It will be hard the first times to find back the sensations but it will come back really soon.
Don't hesitate to read again some theory not to forget or you can even stay tuned on apps that talk about horses, techniques and actuality.