I don't have a spiritual answer for you, but I do have something else to offer.
It's okay to not be positive all the time. It's okay to feel fear. It's okay to feel anxiety. It's okay to be sad. It's okay to be ANGRY! No one can keep this magical positive attitude all the time - no one. Even cancer patients get out the wrong side of the bed sometimes... And you know what? That's okay.
So, give yourself some permission to have bad moments... and have your bad moment! Then, carry on....
I manage to have a positive attitude most of the time - but not all of the time. My biggest piece of advice is show your sad/angry/anxious side with ONLY folks you deeply TRUST. Some of my friends would admonish me for not keeping a positive attitude if I even let a wee bit of anxiety show through... But, my true friends and most loving family members... they could listen... and that was it - they could simply listen and simply be present for me when I needed them. SOOOOOO grateful for those friends and family members.
GOOD LUCK - and a huge hug.