Perhaps 'dominating' wasn't the best word for me to use. My husband isn't dominating either, but when he is with us we have a different kind of sex than when my gf and I are alone. She and I do things when it's just the two of us that we don't bother to do when he is there, if you catch my drift...
I know you say you are 'slower' sexually than he is. Perhaps one on one dates between you and her would help? I know I felt a bit lost when my gf and I started having sex, it wasn't so clear cut as having sex with a man (I'd not had any real girlfriends before her) and you need to figure out what each person likes.
I'm not saying you need to 'keep up' with the pace of their relationship, but you don't want to neglect your private time with her. Otherwise in a month you may find yourself left behind as she and he build a full relationship and you and her only have a friendship.