Reason for Celestia to turn Corrupt

So I was going to write a thing…
 
 
When I realized I don’t actually have a reason for Celestia to have turned corrupt in my little Queen-Luna-verse. I even went back through some of my old RP posts to see if I’d ever mentioned why, but it looks like I never did.
 
So the question is, what are some reason for Celestia to turn corrupt? I don’t want to do a role swap of her being jealous.

Loss of Twilight Sparkle.

@luna the great!  
What is the time frame on this? If its back when Luna would have gone corrupt with jealousy, then I would recommend Pride as her start of darkness. She believes herself to be the most powerful, most beloved and thus that she should rule utterly.
 
She starts to actually think of herself as a god.

Hi! While I’ve decided to go a different path completely than the way Luna was corrupted, I’d like to thank you for your help. The story can be found Here

when becoming nightmare moon, princess luna was imprisoned on the moon for a thousand years. imagine that, one thousand years with nothing to do but stare at the world you wish you could be part of. it was later revealed that luna didn’t go completely insane because she was in an enchanted sleep for the duration of her curse, or some fascimile thereof. sort of a cop-out, but it’s mlp what can you expect. clearly luna’s experience was terrible, but more in the sense of a nightmare than in the sense of a mind shatteringly boring imprisonment.
 
princess celestia had no such magical sleep to rely on.
 
she had to raise the moon every night.
 
reminded of the sister she lost, her only friend left alive.
 
reminded of her failure.
 
for a thousand years
 
every. single. night.
 

@redweasel
 
Ding ding ding, we have a winner.

@redweasel
 
It could be worse…
 
What if she’d accidentally killed Luna while trying to stop Nightmare Moon?

@Scrounge  
Good point.

@Dono & M.Storm  
It’s entirely possible she’d just disappear completely after something like that.

@Scrounge  
Think of it, being stuck on the Moon for a 1000 years. No food, no water, body begins to become weaker and weaker over time. Not able to produce a lot of magic, soon not to produce any magic at all. Exhaustion kicks in, can’t move around a lot, soon can’t even pick yourself up or move.

@Dono & M.Storm  
Or Luna just shatters like Sombra did and Celestia has to watch it happen. Trauma like that would break anyone.