I have a fluctuating condition to.
I'm so scared of being challenged over my badge. Some days it's fine, I'm in a mobility scooter and if I do have to walk a bit I have a very obvious limp and probably look like I'm in pain too (because I bloody am).
But there are days when the pain isn't quite so bad and I can walk a bit. I walk perfectly normally, if a little slower than other people, so if you saw me walk from the car to the shops you'd think I was completely fine and abusing the badge. However follow me round the shop and pretty quickly my limp starts up and I slow down as the pain starts to kick in (and it seems to go from 0 to 60mph very quickly and suddenly I'm in agony). I find my limp pretty embarrassing so I do try and suppress it on occasion too.
Then there are the days where it's not so sore to walk (although they're very rare and usually I've been in bed for a week all day every day for things to settle down that much), but even then I use my badge because I know I'm still very limited in how far I can walk and even if I did get round the shop without things hurting, that evening and the next day I will have an escalation in the pain which won't then piss off for ages, so I know I mustn't overdo it.
It's just not straightfoward. People who steal blue badges to use or park in disabled bays illegally, out of laziness, make it worse for everyone with an invisible disability. Please just because you see someone without an obvious disability, don't assume it's stolen/they aren't disabled. They could well be.