A person with cancer or whatever is weak or ill at all times with everyone. Someone with PTSD acts one way with you and quite another with others. The comparison just doesn't fit.
I'm not sure you are correct here. I'm sure you have had times in your life when you've had some kind of simple illness, say, a nasty cold. And you haven't had the option to just stay in bed for a whole week and rest, relax, and get over it properly. So, you drag yourself to work. Perhaps it's a job you really care about, and you don't want to be perceived as complaining too much, plus, you just don't believe in making everything about how sick you feel. So, you do your best to appear less sick than you really are. And then, when you get home, you collapse, twice as tired as you'd normally be (because you are sick).
Nearly everyone with an illness does this - whether it's a cold or leukemia or PTSD.
Now, if you are lucky enough to have people in your life who really care about you and you trust them, and can really be yourself with them - well, if one of those people were to come over after you'd just gotten through your horrible day at work when you are exhausted and just need to sleep - you would be much more likely to tell that person exactly how you feel, which is exhausted, sick, and tired. You'd tell them because you can trust them, and because you know there will be no negative blowback from being honest. They are your friend, they will still be your friend. And your good friend will say they understand, ask if they can help, you will tell them no, and they will go away and not resent you for not being able to go out to the movies, or whatnot.
But let's say there's something off-kilter in the relationship. Say your friend is insecure about their relationship with you - or say you are worried about ditching your friend because they have been going through a bad time too - any, any number of insecurities, worries, awkwardness - they just bubble up to the surface in times like this.
So, you're the sick person: you can barely stand up - but your friend really needs you so you drag yourself out of bed and go to the movies with them, and the whole time you are wishing you could have just been honest and told them you really, really couldn't. Or come up with some different activity. Something.
Most people will do this: when the friend calls two days later, wanting to go out again, and you still have a cold - most people will avoid talking to the friend and trying to say "no" again. Because we are human, and not perfectly well-rounded walking examples of mental health all the time.
I'm not saying it is right or wrong, it's just what happens, as long as there is some insecurity in the relationship. Make it a romantic relationship and the chances for that insecurity increase exponentially. And make the illness one of mental health (where we are often inter-personally challenged
anyway), and yes, the chances of there being miscommunication, hurt feelings, anger, abandonment....those go through the roof.
And when the relationship is a deep and loving one, those things go through the roof too, for different reasons. When the people we love most are hurting, and we can't do anything about it, it hurts us too, in a different way. They are struggling and we are helpless.
But you can't fix someone elses' struggle. And they can't fix your helplessness. What can happen is both parties can do their best to be always upfront about what's going on, and not be afraid to talk about it. Yes, when you are the "well" one, that burden of being brave can fall on you more often - and that is the challenge every supporter hits when maintaining any relationship to a sufferer.
It doesn't mean you should tolerate things that are intolerable to you. But it does mean that you also can recognize your own capacity, when you are getting burned out, when you need to get some help for yourself, when you need a break.
And on the sufferer side, we have a responsibility to meet you
where we are at, to the best of our (compromised) ability. It's actually the responsibility any person has towards another in any close relationship. Be honest about where you are at and do your best.
Illness is messy.
But don't for a moment think that mental illness is any less debilitating than any life-threatening illness. It is not a terminal diagnosis. But it's also not an allergy. it's not even a broken bone. It's a number of systems in the body going haywire.
I often think, if we somehow showed our insides more fully as a physical manifestation - if the mental illness was reflected on the body, showed up as open sores and wounds and pus and blood - we would be no less misunderstood and even feared, but at least people would see what it feels like.
I don't wish that. But it's the best way I have to describe what it feels like, on the inside.
/end soapbox