I find that with opening, you get used to knowing your job - i.e see off the opening bowlers while scoring runs off bad balls. You also get used to knowing when you are batting, and with the (hopefully) same 1/2 people every week.
On a Sunday I don't open, and have struggled this season despite the worse opposition. I just hate sitting there padded up waiting to bat, and can't seem to get my mindset right. I feel the need to hit runs straight away and inevitably get out because I don't play myself in (as I do when I open).
I also struggle more if people are getting out around me, seems to disrupt my rhythm.