I'm not going to let my girlfriend drink on New Years.
Why don't you want her to drink?
I don't like her, and I want her to have a bad time.
That's a very rude thing to do to your girlfriend. It's a holiday so you should want your girlfriend to have a good time!
People always get a little grumpy around the holidays. So, I think it's fine if I'm a bit mean.
Being grumpy doesn't give you an excuse to be mean to your girlfriend. If you don't feel like celebrating, you don't have to. You should still let your girlfriend celebrate and have a good time at New Years though since it's a holiday.