Hi,
I have had DID for over 40 years and it takes a special person to deal with someone that is DID. My wife has been with me for 37 years and she know my alter, yes, I only have one, and she does not get along with him. She does know who she is dealing with and does well with it but, sometimes she just wants to "make peace".
In all these years she has learned the triggers and she has learned what will work and not work in dealing with me when I "feel one of my turns coming on". Usually she just lets it pass...redirects and mostly helps me get past the episode. It will take time to make peace with the alter(s) and you might not ever be successful, but you can learn how to deal with the "other" in a loving and non-confrontational way.
Best of luck to you and your girlfriend.