Hello, I'm sorry you're struggling. I don't have DID, but I have struggled to be taken seriously. I've found that research psychologists/psychiatrists myself before going to the GP and having the GP refer me to the one I chose is the only way to get where you need to go. GPs are wonderful but they don't know you well enough to know what you're looking for in a psychologist. There are also websites that review psychologists and psychiatrists so can read other people's experience with them and make up your mind from that.
I hope you find a good one who takes you seriously. Good Luck!