Riddle

So, I'm supposed to be writing, but this is what happened instead. (#distractible) 

She steps on the Moon without any light.

She curses at Mars but still it is night.

At Mercury she voices a spirited cry.

At Jupiter she stumbles, and then realises why.

When she’s at Venus, she still carries a load.

No gifts yet at Saturn, it’s not the end of the road.

She’s not pretty or happy in the light of the Sun,

Her path hasn’t ended nor has it begun.

She is numbered like sisters

And heavens and stars

But she is ever alone

And ceaseless is her path.

What is she? 

Guesses?  Use the comment box.  

ETA: (Sat night) OK, this riddle isn't going very well. None of my family members have managed to work it out yet!  Also, I thought it would make better sense if I tweaked the answer a little. (So I did). I'm still not 100% sure if it works, but I'll leave it up here in the spirit of a beta reading. Enjoy!