I have a love/hate relationship with my best friend's girlfriend! I need to get this off my chest.
Oh dear me. Do you actually hate her? If not, in what way do you feel for her?
I like her because she helps me when my friends won't, but she can also be callous towards me when her boyfriend is around.
Since your best friend is very close with his girlfriend, it's expected that you try to keep the peace whenever you can when you interact with her. It's inappropriate to cause drama between your friend's girlfriend and yourself as it will cause a lot of stress for your friend.
When he's out of the way, I'll tell her that I love her.
This is a relationship that needs to be apart, otherwise it will be deemed unhealthy in the long run. Nothing that good can come out of a both good and bad relationship with your best friend's girlfriend.