I don't want my girlfriend's best friend to come on holiday with us
Why don't you want her to go on holiday with you?
She's just too loud. I don't want her to spoil the trip with her bad cheer
Wanting to spend time with your partner without their friends is good. Friendships and relationships take effort and compromise. Tell your girlfriend you want to have time alone on the trip with her.
what if her needs conflict with my wants? What if she really wants her friend there?
Then you two should compromise. Maybe she goes alone with you this time and next time she goes somewhere with her friend.