Katie, I don't think you are a coward. We have all been in the situation you are in now.
Even once I was certain that I was trans, it still took me months to work up the courage to tell my wife. Some of the folks here remember the struggle I had. I had every reason to assume that she would be accepting, and yet I could not do it.
I had to do some hard work looking at myself to get a clear picture of where I would be in 10 or 20 years if I didn't tell her. It was only the horror of that vision that convinced me that I had to do it. And I had to fully accept within my heart that I would be all right if my wife decided to leave me. Not to neglect her feelings, I had to decide that I would make it as right as I could for her, financially, if she were to leave. Once I had accepted those risks, I was able to do it. And it was still the hardest thing I have ever done.
I am not telling you my story to scare you. What I am saying is that this journey we are on is hard work. You are not a coward. You are just not yet ready. But, keep at it and, in time, you will be ready.
In the meantime, as you do the work you need to prepare, be kind to yourself. You are human, and you are facing a scary road ahead. It is okay to be worried.
And, sometimes, in spite of our fears, things just work out. The scariest moment of my life was when I said, "Sweetie, there's something I need to tell you." Two minutes later, I heard the most beautiful words in my life: "Whatever you decide to do, I will support you."