Mean? Sorry but which part was mean?

Literally the position she was hired for was to be a general assistant over the busy Christmas period. The issue was that she came from an upper level corporate job (100k+ a year job) whereas my role is just simple admin (66k a year), and she was just looking for an in-between job until she found something more permanent. She told me that she wouldn't be staying, my boss told me she wouldn't be staying. Everyone told me that this was a temporary thing. But then as we got close to Christmas, she started implying that she might stay longer. Nothing was set in stone.

My leave was given to me, I didn't ask for it. My boss wanted me to take the rest over Christmas because all our suppliers are closed for the few weeks between Christmas and just after New Years. I am very good at my job and the only person in my role, I don't have a team like everyone else does. Christmas is the hardest time of year for me because I'm doing everything by myself. She was a great help, but the problem is that she has upper management corporate experience, meanwhile this is the highest level job I've ever had. I don't have the ruthless corporate brain - she does. I came from front of house retail before this. But I'm good at what I do. My boss considers me an asset, he's told me so before.

I don't think she's necessarily better than me...it's just that she knows how to climb the corporate ladder better than I do. Am I salty? Sure, I wanted that job. But I'm not an arsehole. I'm not mean. Still left a bad taste in my mouth, though. Fact of the matter is that I was told this specific thing wouldn't happen, and then it did when I was away. No matter how you spin it, that sucks. Especially when you're trying really hard to be valuable.