Correct me if I’m wrong, sounds to me you are someone who has a lot of emotional issues/baggages. You have clearly learned to adapt in order to… protect… yourself. You don’t trust anyone. You have already developed a strong shield and feel it’s you, you alone against the world and so far this strategy has been working well and you feel in control.
The problem is this secure and comfy life/marriage is making you feel losing control. (you need excitement and obstacles, it makes you feel alive and by overcoming those makes you feel in control)
Many people would say emotional issues need to be resolved whereas I believe sometimes it is just too late for that. It’s very hard to change people over certain age. As long as you have found a way of “covering up your issues” and found a way to make yourself happy then that’s good enough. But it’s very important to be aware of your own issues so that your action won’t have a detrimental effect on the vulnerables. In this case it’s clearly your child.
I believe it’s not your intention to leave your husband just to marry J. As you said J was just a trigger for you to realise that you haven’t been happy. All I can say is do whatever makes you happy, becasue in the end people like you will always do whatever makes themselves happy. But maybe do consider giving the custody of your child to your husband as your definition of a happy life for yourself may not be the best option for your child.