This is going to sound ridiculous but ride a bike and also watch other people ride and continuously watch people riding in videos if you're nowhere near a barn
Definitely try and keep fit and well when you cannot ride but the best way to stay on track is to have a second pair of eyes. I totally recommend having a lesson as your first ride back just to make sure you can remember everything :)
Also, don't be too hard on yourself!
Maybe work on your strength and balance in the mean time? I never liked yoga but I did find it useful in my riding as it strengthens the similar muscle groups I use when I'm riding as well as improving my balance and body awareness. Swimming is good too!
I couldn't ride for 6 months after a bad illness so I know how you feel. But honestly it's like riding a bike, once you're back on you know what to do. What frustrated me was what my mind wanted to do and what my body could do were two different things. So take it slow and build it up. I was actually nervous when I got back and felt super high off the ground which I didn't expect. But within a couple of weeks I was back to myself and now I'm better than ever! But I frequently took breaks during lessons and made sure the instructor knew when I was done or struggling! Hope this helps!