Your boss defiantly over stepped his bounds. Even if he was genuine and caring, perhaps he should have not pushed you. You are still responsible for your response. I would go to him soon and tell the the truth, and apologize. Tell him u just got scared, overcompensated and this is a deeply personal decision. Thank him for caring and ask again this be kept in confidence.
Also want encourage you to examine your motives for worrying about others reactions do much. You are doing this for you. Show confidence in yourself and your decision. Most people will back off and respect you.
About 2 weeks before surgery I told everyone I could at work. It was the most freeing thing. No shame, no stories. I know that may not be your answer. But it did help embrace the new me.
Congrats on starting a new life. You will be awesome and do great.