I am so sorry he has not been supportive or understanding of this decision, I know how it makes a huge difference when the people we love put effort into understanding us and being there for us. It seems that he does not see the value in having a more confident, happier, healthier girlfriend and what a difference that could make in so many aspects in your relationship, your life, your mental and physical health.
This decision is not about him, it is for you and about you; believe that! Please do not feel guilty or that you owe him anything from what you chose to do for your own health.