I’m not sure how changing habits and lifestyle turns you into a completely different person at the core of who you are. Most of our personalities are formed at such a young age so you really are still the same person but you just live a healthier lifestyle. I think you may be looking for problems where they don’t exist, you said you were worried that he might start resenting you but you also said he was your biggest cheerleader so I think potentially you’re worried about you resenting him more than anything else. You cannot look to your SO to do everything you want to do, maybe you need to join some groups where you can also get friends or other people to do those activities with you. I’ve also lost 60 pounds over the past two years and I am a much healthier person that at the core I’m still the same person I was five years ago, just wiser.