Oh well, DP got sick when I was, hm, 19 so I've been doing this a long time. I was mainly a lot more rubbish and not clued up, but you do anything for ten years and you get (a bit) better at it. 
Yes, that's the thing about any chronic injury: people can't really grasp it. What, you're still in pain? Again? Even when it's inconvenient? Even though it's Christmas?
Empathy burn out, weirdly, happens to more empathic people, I think. I don't have good empathy, only lots of willing. I try to go off what people tell me about their life, rather than my own insight, because my insight is pretty poor. DP has learnt to be explicit about what he needs from me: a drink, to be left alone, attention, etc; he does not have the luxury of having anything anticipated. The flipside is that he does get his drink/backrub/attention. Articulate your needs, I am always begging him!
I think it's hard to do that, especially if you're a Guesser. If you're a Guesser, I observe, you also do a lot of guessing about what other people want from you - you are constantly engaged in a very complicated negotiation about what it is okay to express. I have no advice there, haha, sorry. I am an interpersonal ingenue. But the hot/cold therapy, that I can advise. Cheapest at Medisave.