Can we ever truely accept the impact of our illnesses?

So, I went back to work on Wednesday. I worked Thursday. Today I slept through my alarm after a difficult night and I am totally exhausted.
I have had to realise that I was quite unwell when I originally started my placement, and that is hard. Will I ever accept it? Or will I keep battling on and being crushed every time I realise that it does have an impact on my abilities, and no matter how much I try, I have periods where I find it extremely difficult to function.
I am so tired. I feel like a huge failure, because I am unable to cope with what everyone around me seems able to do. 37.5 hours a week, that is all. I am exhausted after two days. I was tired after just 4 hours. I don't feel low as such, just negative, but I think that is because I am watching myself fail all over again. I know this placement is not going to last forever, I just need to do the hours and get it finished, but all I want to do is sleep.
I'm always more tired than my friends and my collegues. I don't know if this is the condition or the medication. I really feel quite hopeless. I just want some energy.