The Tide
Here's an observation on sickness:
Everyone assumes you're fine until you show them that you're not.
If you have depression, you can go as far as explicitly telling someone you're depressed, but they won't believe you until they see you acting depressed. Until they see you sleeping all day, letting the dishes pile up, and going extended periods of time without showering, you're fine. Because of course you are.
I have a chronic sleep condition that results in excessive daytime sleepiness, in addition to a number of other symptoms. It's an extremely crippling condition, but medication, caffeine, and stubbornness can make it somewhat manageable. So manageable, it seems, that people often forget that I live in a state of constant exhaustion. I was once told by my doctor that living with this condition means I am constantly operating as if I hadn't slept in 24 hours, even when I've had 8 hours of sleep. Medication can only do so much for that. But even though I'm constantly telling people that I'm tired, no one seems to believe me. No one seems to believe me, that is, until they need me to show up somewhere at 9AM and I slept in. No one seems to believe me until we make plans and I have to cancel because I'm too tired to safely operate a vehicle. No one seems to believe me until they can't get in touch with me for days because I've been asleep almost the whole time.
Oh, and then of course there's the period of time after one of these periods, when people think you've gotten over it. As if chronic health conditions could just go away forever. When the depressed person starts showering again and regularly leaving their house, they're not depressed anymore. When the hypersomniac stops sleeping 16 hours a day, they must be getting over it/growing out of it/healed/cured.
Hint: they're not. These things don't just go away. They follow us. Each and every last person on the planet who struggles with chronic health conditions that come and go like the tide knows what it's like to live in constant fear of the water. None of us want to drown, but we know the tide will always come back for us.
It's so easy to forget that someone is sick when they look healthy.
I write all this to say that if someone tells you they're going through something, whether you can see the effects of it or not, they probably are. Don't let yourself forget.
Remind yourself that there are invisible illnesses. Be kind to those in your life who have trusted you enough to tell you they aren't always as okay as they seem.