There are a few things you can try and the answers you get may clarify things. Ask if there is anyone who knows more about what's going on than everybody else and ask if they'll have a conversation with you. Asks them if you are sharing the same body.
If you do have DID and another alter takes control of the body and you are aware that this is happening more or less, this would happen if you are co-conscious with another part of you.
It's quite preposterous for a therapist to doubt their client's communicated experience is real so I'm wondering if they actually said "I don't believe you" as in "I don't believe that you're telling me the truth." Or did they say or mean "I don't believe that's what you're experiencing." Either way, you'd need to find a therapist who doesn't give you that impression.
Are your reading several threads on this board? It's a good way to see if the experience of others here matches yours? For many people with DID, starting to read threads here is incredibly reassuring. Hearing others express experiences we have is quite validating.