Staying Hard While Bottoming

So I am fairly new to bottoming! I want to be a totally submissive bottom and be used till the end of time! I want to be fisted, I find it so freaking hot! However as I practice taking sliding up and down the bigger portions of my toys I find myself getting softer and softer! Still turned on for the most part but no excitement in my cock! Is this normal? Is this just a fantasy I will never be able to take part in? Is this just a learning curve?