<human>: My British girlfriend and I have had a good, strong relationship over the past 7 years. She is extremely generous and I love her to bits.

Unfortunately, she has a mean streak. It seems after any social event we go to, she actively looks for the bad qualities in people and/or tells me all about how a certain person pissed her off. At the end of every night we go out to meet with friends there are only bad things to say about others. Now this in itself bothers me immensely as I am the polar opposite and enjoy seeing the best in others however this mean / grumpy side of her also gets projected on to me. 

Frequently, I will come home from work and she will be in a bad mood. I have to work my arse off to cheer her up. My success rate is 50 50. I am at a point where I don't think I can stand to put all my time and effort with her trying to make her be a little positive about the smallest god damn things. It is incredibly draining on our relationship and her being in a bad mood just gets me down.  From my perspective and I am at a loss for what to do. At the moment this issue is going to be what breaks the relationship. 

I don't understand where this comes from and I am in this conundrum where even if I flat out tell her I don't like it that only makes her even more grumpy and mean. How do I help her transition to being a nicer without me coming off sounding like I am going for a full character assassination?

Any advice is greatly appreciated!

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: My girlfriend is grumpy, mean and cynical about others to my face and I don't know what to do about it.