Four in the morning and my pain has finally dropped from severe back down to moderate. I feel tired in a way that has very little to do with needing sleep.
Anyway. Here are some thoughts on it, plus previous experiences. Going to use third person for them.
Being alone and in severe pain is kinda miserable. They don't have the energy or focus required to think about something else or do anything but endure and wait.
This will probably put them in a bad mood even if they're used to pain. It's just annoying.
Having sympathetic people around makes it much better. Sharing stories about painful experiences can actually really help because they don't feel like they're trying to do the impossible task of ignoring the pain but they're also doing something nice/fun in interacting positively with people.
Sometimes it's bad enough or they're tired enough that all they want is someone to sit next to them and go "I know, I know, easy now. There we go. Just breath. It will pass."
If no one else is available they will do this for themselves.
When pain spikes they might bite down on whatever is available for a bit, including their own arm. They might also throw up.
There's a decree of harshness with themselves that comes from being used to it. They'd be upset if anyone else told them to pull themselves together but they'll say it aloud to themselves when they have to do something and have the urge to just curl up in a little ball instead.
If they're not too tired, they'll frequently shift position to try and get more comfortable even though they know it won't work. It's like the subdued, casual version of writhing in pain. The same animal urge to get away, but without the intensity.
They'll tremble most strongly in the moments when the pain isn't as bad, because when it is they'll probably be tensed up.
If they're really used to it, they'll be less tense.
They might expect their heart rate to have sped up and be surprised when it hasn't even though they're breathing raggedly.
They may feel the constant low urge to cry or scream but have enough just control of themselves not to and think it would feel childish- again, even if they wouldn't judge others for expressing pain that way.
They'll probably remind themselves they've had worse (assuming this is the case, though some might say so even if it isn't, or not realise it was the worst they've had until later, because all severe pain feels like "the worst" when they're in the middle of it).
After a while they'll probably get impatient just waiting for the pain to pass and try to do something. This will often make it worse. Depending on how stubborn they are, they may push through anyway, do it but slowly and with breaks, or decide against it after all.
As they get more tired, their pain tolerance will go down. Since pain interferes with sleep, this can lead to a continuous problem.
They might switch between being angry and being gentle/comforting to themselves from moment to moment.
They might also have moments of bravado or a sort of strange delight where they're just grinning though it instead of grimacing. This tends to happen much more often when other people are around. There can be a fierce sort of pride in knowing they're taking it well.
Once it abates, even if it's far from over, they'll probably be very tired and just want to rest for a bit. Or they might feel energised by the relief, especially if it hasn't been for that long.
If they're in pain permanently, then being pain-free is genuinely blissful.