My 10 year old daughter has temper troubles and behaves very rudely with my girlfriend. how do I find out what’s wrong?

she is insecure of course. i think the fact that your girl friend is going to become a priority over her is a big threat to her, as well as the fact that your girl friend is taking away the spot of mommy. talk to her, and be genuinely interested in what she has to say, without losing your cool. spend as much time as you can with her, on a weekly basis. show her that you love her the most and no matter who else is important, she will always remain your number 1 love :) this is a time of change for her, both in terms of the tween years as well as life changes, so this kind of behaviour issue is normal. all the best to you :)