I am one of those who thought they completely recovered but then realised life had different plans.
I know it's exactly what you're asking: How to NOT fall back into "the void".
But I'll be honest and share what someone just told me about Ups and Downs.
You can't avoid getting worse again at some point.
As long as you're living in fear it might happen, a part of you already experiences it already.
It's similar to weather. I can't control the weather. But if I hide because at some point it's going to rain again, I will also hide from sunshine.
Instead I can try to embrace the good weather, be glad, thankful, relieved, happy or just "at least not in a crisis"; and at the same time I can prepare for weather changes.
Better clothing, an umbrella...
I believe the key is to have all the tools to challenge those times. Not to completely avoid them.
(Speaking as someone who lived in fear of becoming worse and not recognising my way downstairs again).
A good start could be to brainstorm about what helped you in the past to "survive" the hardest times.
What mindset did you have?
In what activities did you engage?
Who did you spend time with (and who not)?
And these things don't have to be "the big solution".
In my case, it brightens the day when I look at cute pictures of animals.
Or I feel refreshed after I cleaned my teeth.
Opening the window, letting fresh air in.
Preparing for bad times can be...
Make sure, you let people who are close to you know.
Have all needed medication stored (skipping them can cause an even worse crisis)
Try to focus on the bare minimum and human core needs. If it's hard to drink enough, prepare enough bottles and place them next to the bed.
Buy food that won't go bad fast or is prepared easily.
And so on. Everyone is different, and recovering is as well.
But I've seen many reporting that fulfilling "the basics" helped them to battle tough times.
I truly hope this post doesn't encourage you. "Bad times" don't mean necessarily a crisis again. It can be "just a bad day" you have, but manage to not fall completely back.
But life isn't only the one or the other opposite. No matter how we wish it was the case or how others seem to live their perfect life.
And never forget: setbacks are normal. We're not machines (even machines run into errors and have bugs ;) ). Giving the best looks different day to day.
we don't have to give the best everyday anyway. Some days just need to be survived. And that's great! That means, you still go forward.