How do I motivate myself to write? I start with the " I'm gonna do it " attitude, turn on my computer, write 5 sentences, and leave it there? Ideas are floating in my head, great ideas, but I just can't get them out

Is it possible that the ideas in your head are only fragments of an idea? When you have an idea jot it down immediately. Then as you continue to run that idea through your mind, add any thoughts that might enlarge on that idea. Gradually turn that idea into a scene. Ideas come from a different part of your brain than the act of forming them into sentences. Some people can do both at once, many cannot.