Have you tried asking him what it is about what you used to do/type of person that he doesn't want you to return to? If it's returning to being a fitness instructor this is the best thing you can do for yourself physically, and mentally. You say he doesn't want you to be the person you werre....this is a little vague so I'm not sure how to respond to that. If you were a good person, and had a passion, why would he not support you in this? Maybe it makes him feel a little insecure, and you two need to talk about this. You're vague in explaining the situation, but you have to do what makes you happy and he should be happy for you. Take care and I hope the two of you can work this out.
this man may be nice but what was you like before you met him that he doesnt want you to return to,if it health worries then he got a point,but if it doesnt affect you in that way then you crack on and live your life how you want too,we only gets one shot of life so grab it with both hands if your man truly wants to be with you he will except your choices in life and support you