It's good that your Master allows for communication so you can share your feelings with him. Ultimately, you do have to trust him, but it helps if he supports you all the way.
Did you ask his reasoning behind wanting her alone? Are you allowed to ask? It's possible he feels it is best for her, maybe if you put yourself in the situation of the second woman you will understand better?
If he is a caring Master, as you seem to say he is, he will want to take a good care of both of you. He probably needs to reassure her as it will be her first night and probably overwhelming. I don't think he has any intention of having you come last all the time.
I hope you can find an arrangement that won't be so hurtful to you. He is your Master and you must obey him, but it's also his role to give you orders that won't end up hurting you (well, depends on what you're into, but you know what I mean).
I think it's really good that you guys communicate, as it can be harder with a D/s dynamic, but communication stays very important, you can't read each other's minds, nor should you be expected to.