See, Lily. It seems you're missing something. Maybe it's just the habit of sorting things into rules and roles that's confusing you. I'm a Switch, but not because I'm one day dominant and submisive the other. Being a Switch to me is just being "free" from the conventional rules. It's just having control over my own needs and desires. If in the middle of a session I feel like being submissive, then I naturally and gradually "Switch" to the other role. Sure thing is that your partner will have to be a understanding and patient one, so that he won't lose all the fun. Many times I used to do it without even noticing what has hapened. Mostly I had my (ex)girlfriend wearing an outfit that would fit in both roles, so that the play didn't lost it's flavor.

Anyways, If that's not your case, then you must be, like Mistress Florence indirectly sugested, in a lack of imagination. It takes more imagination and understanding of the other to be a Dom than most think. If you're willing to have some atention over you in the middle of the scene, then you must imagine a way to do that and still stay on your heels (sorry, this is an expression we use here in brazil). Or keeping your role and posture. As long as your imagination doesn't fail you, it will be allright. If it do, then take your time, put him over your lap and do something more simple until you can sort things out.

Hope to have helped.