If your husband had any qualms about not talking about his hobby when you asked him not to, he should have said so at the time of asking and initiated a calm discussion; the fact that he agreed to your request, then did the opposite and only let you know by accident says to me that he has lied to you deliberately and is not to be trusted. I completely sympathise with your feelings about this, and am sorry that so may other responders have chosen to focus on what they imagine is your attitude to your husband's hobby, rather than the likely repercussions for you from your bullying cousin learning of this and choosing to be unpleasant about him and therefor you.
Apparently #BeKind only applies if you've reached the end of your tether and killed yourself, so my advice (weird as I'm responding myself!) would be to seek the advice of a professional stranger who will listen, rather than risking your mental health by exposing yourself to the judgement of people who sound very much like your cousin. Marriage counselling can be undertaken by one person, for instance. It might be helpful for your to have rather more space to explore how these events have affected your feelings for your husband.
There are many different sorts of counselors and therapists, even amoung those who specialise in one field, so it may not be easy to find one that you feel comfortable with, especially now, but it seems to me that you have worked very hard at your marriage over the years, and want to change how you feel at the moment, if that is possible.
If nothing else, you deserve to spend some time unraveling the complexities of your current feelings and finding some peace and contentment in your life, and I think that doing so will help you to feel more confident and secure in your decisions in the future