I think to a certain extent you do have to keep up the optimism. My brother has cancer and on top of everything else he has to comfort people and reassure them about how he is coping and that sucks. I think that it's important that if you are his main support person that your DP doesn't have to comfort you.
On the other hand there is not much point in telling lies just to sound upbeat - it's a really tough balancing act and you need to be so strong

Please don't apologise for posting about your DP so much. It's huge. And you need support to be a support so please let us support you. Keep posting whenever you need to. Whatever you need to - be angry, be sad, be cynical, have a laugh. It's all ok.
And if you need extra privacy to say the really ugly stuff there is the grief forum.