I've lost all respect for my girlfriend.
Why would you say this about her? Why don't you respect your girlfriend?
I noticed she shares her time between my friends and me.
Relationships require a lot of work and you need to respect them in order to be with them. If you can't respect your girlfriend and don't love her you shouldn't be with her.
I'm going to break up with her because she has bad manners.
Usually someone would not continue a relationship with someone they no longer had respect for.