First of all, thanks for sharing your story with us! You might think that being afraid of the dark is a childish thing, but you would be amazed to find out how many grown ups share the same exact fear! Truth is, when it comes to fears (and emotions in general) we are all quite "childish". That's because of a very simple fact about fear: we can't control it! Not being able to "control" something, as we do with most details in our everyday life, is a quite frightening experience. We're used to control, to stability, and fear breaks all the rules. It makes us feel powerless, like our hands are out of the steering wheel.
So, how can we cope with this? Can we fight fear? Can we try to contain it? I bet you tried all of these strategies (as I, and everyone did) and found them to be very inefficient. That's because fear is an automatic response from our brain. It's something that's meant to be instantaneous, in order to allow us to have better survival chances in a threatening situation. After all, how useful would fear be if you had to THINK about how dangerous a tiger is?
So, if we can't really control fear, what can we do? Well, accepting that fear is an automatic response and that we can't control it is the first step. Knowing that being afraid doesn't make us better, or worse, people, is also important. Sometimes we feel guilty about being afraid and that worsens things, it makes it a tabu, something we can't share or talk about.
After that exposition is the way to go. If you slowly expose yourself to darkness you'll begin to understand how fear works, and you'll be able to better cope with the symptoms. I know that feeling your heart racing and your limbs trembling can be really scary, but if you put things in perspective you'll find out that these are only physiological reactions to prepare you for fight/flight. The more you get used to them the less they bother you.
You can try to stay in dark rooms for small amounts of time, embracing fear whenever it comes. You should only leave when you are calm again. As soon as you feel comfortable increase the amount of time you spend in darkness. Be ready to be bombarded by all sorts of thoughts, but know that these are only part of this defense mechanism. You can name your thoughts out loud, this helps bring them to reality, making it easier to disconnect from them.
Most of all don't let fear be the center of your life. You have in you the capacity to know your fears, to accept them and to make the choice of not being dominated by them. In a way it's a training. Like you would train a sport or a skill. But here you're training to know yourself (and your fears) better, so you can learn to live alongside them!
Hope this helps a bit!