When you're sick for a long time (and I mean in a 'confined to bed for months' way), you kind of loose the ability to push yourself through things. Not that there is much you can do - after all, there are plenty of illnesses where pushing makes yourself worse, so you have no choice but to become resigned to making the most of a sedentary life.
I've been getting so much better over the past year. It's truly amazing, and I am so thankful to God. However, my 'pushing' muscle has really atrophied. I keep working on building it up, and I am able to get through a lot more now than I used to. So that's good.
Still, every now and then I get laid up with something and the muscle reverts back to the resting state. I recently recovered from a week-long migraine, and the thing about migraines is that you cannot push yourself or they get worse. So I had to spend a week going "yes I could /maybe/ do that but then I'll have a relapse so I'd better not..."
Migraine is over now. However my body hasn't gotten the message that it's time to get back to work. It's really, really hard to push myself to do the hard stuff. I've gotten lazy. I have to not beat myself up over this, but just do my best to build up that muscle again and get things done.