Boyfriend's mental issues

So I'm with my first boyfriend ever. I'm young. We've been together less than six months. He's altogether such a great guy. He's funny and so sweet. I absolutely adore him. He cares about me and loves me so much.

Anyways, the few problems I've had with him are when he switches to different personalities. He acknowledges these. He told a girl he liked her but wasnt going to act on it because he's with me (which I didn't know about until she told me). I confronted him about it and when I read the texts that he sent her (with the girl's permission) I felt as though when he was telling her this he was in a different personality. He sounded more sad and depressed. He didn't sound like his normal self. His normal self, which is mostly around me, is more cheerful and lively. This guy was sad and confused. He also goes into a personality type where he gets really mad or angry. He looks more annoyed at the world than normal. He doesn't take action on his feelings, but he's subconsciously snappy.

When I confronted him about why he said those things to the girl he said something about his memories that he has that arent his. Like he acts on memories that he never experienced. When I ask him about them he says he doesnt remember. I dont think he'd ever lie to me to make me worry.

I dont know what's up with him and I just need help. I want him to be safe and happy. I truly think that the two times I've felt bad about our relationship.is because of the personality thing. Thanks for reading.