Maybe Im a bit late here girl, but you dont need to leave him to change. You need to find your old happy self and revert! Just be yourself.... then the decision will be easier for you both, and more clear I think. If you are naturally a more positive person than be more positive, dont let HIM effect your mood. If he is still always negative, and your postiveness has not worn off on him, then it will be clear to you what you need to do and easier for you to do it cuz you'll know and remember your true self! Just be true to yourself!
(((HUGS))) Ive been there! I was not a happy person with my ex and I never left him either... but then I started standing up for myself and demanding for him to give some of himself to me for once instead of me always giving and HE walked! Oh well... honestly Im better off w/o him bc for 10 years I gave up my own life to live his! I mean it was to the point where I couldn't buy like adidas sneakers cuz he hated them! Ya know??? HAHA I realized that he was doing EVERYTHING he wanted, living the life HE wanted, never making room for me, or consideration for me and I was doing all the back burner living so I could keep him... UUGGHH!! I missed out on all the things I wanted in my 20s bc I did all the things HE wanted!
Oh yeah, so all in all... living HIS life was making me miserable and making me be a person I didnt like, never mind others! I was not a happy person bc I was confused on WHO I WAS!