It sounds to me that you may be having problems with the personality to be a Domme - you say you need to get angry to do anything to him and that means you sometimes overstep the bounds. Maybe you need to consider a different style of Domme. You say you like the idea of gentler Dommes and that is obviously a way you need to take this. You need to create a personality, play a role with him as if you were an actor. You are no longer his sub (which may also be a barrier to you domming him) when in this role but a Domme. I also like the idea of a ritual to indicate the switch but you can also consider props or costume to assist it. Something as simple as 'when I am wearing the black leather gloves I am a Domme'. Try and think about how this character thinks, acts, speaks, walks, everything. There are Dommes (some professional and some lifestyle) of my acquaintance who do this very thing. If you talk to them in normal conversation - either on e-mail, phone or in person - you can have a perfectly normal, everyday chat to them, just two friends rather than Domme and sub. They are still usually Dominant personalities but not to a significant extent. However, when they are 'in session' they turn on the 'Domme personality' and become more or less a different person.

It maybe worth speaking to a reputable pro Domme for advice on how she does it?

I would also like to add - communicate, communicate, communicate... you talk on e-mail/phone etc and that is good but take it a step further. Order him on your next phone call to send you a description of his greatest Domme fantasy (obviously with no internet connection he will have to send this through the post) and maybe tell him to complete a BDSM checklist. Both will give you a pretty good idea of what he wants and allow you to feel a lot more confident about it all.