Why can I never just sit down and make myself write?
Don't feel bad at all if you have trouble making yourself sit down and write. I think this is completely natural and this happens to a lot of writers from time to time. It's good though to get into the practice of overcoming your mind and making yourself write something to walk yourself through it. Just the mere practice of writing alone can develop into a concept or idea.
Procrastination can be one the biggest obstacles in a writer's life. If we keep putting off our own writing needs, then a week can turn into months, and months can turn into years without us even realizing. When you feel the urge to write, grab whatever is by you--- your phone, your computer, a napkin, or paper and write something. Even writing a sentence can kill off procrastination.
The number one excuse I hear when writers come to me is that they "don't have time to write", but they want to so bad. I understand we are busy playing different roles in our lives. We are parents. We are employees. We are business owners. We are spouses to loved ones. We have a lot of daily responsibilities that we are in charge of taking care of. However, need us not forget we are also human beings, with an ever constant "thinking" mind with trapped thoughts that are yearning to find an outlet. The time you spend quietly thinking can be the time you spend quietly writing.