A girlfriend, not a mistress
I feel for you. You are in a difficult situation. I have a few thoughts.
First, you don't have a mistress. You have a girlfriend. A mistress is someone you cheat with on your spouse. They are a secret, something hidden from everyone. Your husband seems like he wants you to treat Missy like a mistress because it is easier on him. Don't go this route. Yes, you cannot be as open as you would like with him right now but don't, even on the intertubes, call her your mistress. She is not. She is your girlfriend.
Second, you write that you are attempting to behave at home exactly as you did before getting involved with Missy. This is impossible. You are not the same person and so will not act the same. I assume this is your first serious relationship with a woman? Something major has happened and you have changed. You cannot put that genie back in the bottle no matter how hard you try. You can agree not to bring her up or talk about her for a while but behaving like she doesn't exist is impossible. He should not expect that and you should not agree to it. That is an unreasonable expectation on both your part's.
Third, it seems like your husband was fine with your interactions with others as long as they were meaningless, and/or he could watch or participate. You didn't have any meaningful connections (by meaningful I mean more emotional or romantic involvement) during your marriage up to this point. So he apparently didn't believe you were really bisexual, as in capable of having a romantic, emotionally invested relationship with a woman. I realize you told him this but your behavior gave another impression. This is not to find fault with you! You were happy with being monogamous for a time or having sex-only outside interactions. You did nothing wrong but I can understand how your husband may have felt blindsided when you felt a deep connection to Missy. (It is a sexist but not uncommon attitude held by some men that women having sex is hot but two women having an actual relationship is threatening.)
That said, does he understand that he is going to have to do some deep, intense personal work? He cannot shunt this off on you indefinitely. He cannot refuse to work on the underlying issues of why he is threatened and expect the marriage to survive in any real sense. Does he understand that the marriage is at stake? You clearly do not want to leave him but this situation is hugely stressing your marriage (and your relationship with Missy).
Now it is new - just 2 months ago. That is not very long. He does need time and attention to start dealing with a new and different reality than what his marriage has always been. A DADT can give him space to start learning, examining himself, and coping with new realities. But it should not be an excuse to hide from the truth, to not deal, to expect things to be as they were. That reality does not exist anymore. Derby's suggestion of a time frame is a fine one. And you and he will have to talk at great length over and over about this new reality. That doesn't have to happen right now but it needs to happen at some point.
I wish you the best.