I'm going to criticize my girlfriend's decisions
How would you criticize her decision?
I will say she spends too much on frivolous stuff.
It is fine being concerned about your partner, but you should respect her as well. Make sure when you do talk to her that you aren't disrespecting her independence or decisions.
I am going to say she is a gold digging, materialistic and selfish bitch.
I'm sure that's gonna hurt your girlfriend and negatively affect your relationship with her. If you don't like her just break up with her but there is no excuse to use this kind of language.